Guide to preparing for a half marathon (21k)

Preparing for a half marathon is one of the most common goals among runners who have already conquered 10K and 15K distances and are looking to take the next step in their evolution. The 21.097 kilometers (13.1 miles) represent a demanding combination of endurance, strategy, and mental preparation that requires planning.

The first half marathon is a turning point for most runners. Its distance of just over 21 kilometers demands discipline and consistency during preparation, but when completed for the first time, it generates great personal satisfaction and, above all, builds confidence.

Over the years, I’ve coached many people who, after completing their first half marathon, confessed to me that they finally felt like real runners.

Preparing for a half marathon isn’t just about racking up miles; it’s about structuring your training, respecting recovery, and understanding race strategy. With proper 12-week planning, it’s possible to reach race day with confidence and control (and I want to emphasize that we’re referring to people who already run 10K and 15K comfortably and therefore have a training base that allows them to run regularly; we aren’t talking about people starting from zero).

One of the most frequent mistakes is increasing training miles without criteria or structure, which usually leads to excessive fatigue or injuries. In this guide, you’ll find a progressive and realistic approach to prepare for your first half marathon with confidence, minimize risks, and reach race day in optimal condition.

How much time do you need to prepare for a half marathon?

The time needed to prepare for a half marathon will depend a lot on your starting point:

• If you already run 10Ks regularly, 10-12 weeks may be enough.
• If you run occasionally, 12-16 weeks.
• If you’re starting from zero, it’s best to first consolidate an aerobic base for 6-8 weeks before starting a specific plan to prepare for a half marathon.

Recommended frequency of training days

One of the most common questions when preparing for a half marathon is how many days a week you should train. There isn’t a single valid answer for all runners, as it depends on level, experience, and race goals. However, these are the most common frequencies for preparing for the 21K with confidence.

3 days per week, if you want to finish feeling good

Training three days a week is a very suitable option for runners facing their first half marathon or who simply want to complete the distance while enjoying the experience.

With this frequency, it’s possible to develop the necessary endurance without an excessive training load. It’s usually structured with:

• 1 easy or recovery run
• 1 quality workout (fartleks or tempo)
• 1 long run on the weekend

This approach allows for safe progress, minimizing the risk of injury and making it easier to balance with work and family life.

4 days per week, if the goal is progressive personal best improvement

Four days of weekly training is probably the most balanced frequency for most recreational runners who want to progressively improve their performance to prepare for a half marathon.

It allows for more variety in the plan and working on different capacities: endurance, race pace, and running efficiency. A typical structure could include:

• 1 easy run
• 1 quality workout (intervals or tempo)
• 1 controlled or progressive run
• 1 weekly long run

With four days, it’s easier to consistently rack up miles, which is key to arriving well-prepared on race day.

5 days per week, if you have a more competitive goal

Training five days per week is usually aimed at experienced runners looking to optimize their performance and significantly improve their half marathon time.

This frequency allows for an increase in total training volume and better distribution of loads, incorporating specific sessions such as:

• intervals or repeats
• controlled pace runs
• threshold workouts
• more structured long runs

Of course, as training frequency increases, it also becomes more important to take care of recovery, rest, and injury prevention.

The key to preparing for a half marathon: consistency over quantity

Beyond the number of days, what’s truly important for preparing for a half marathon is consistency. A realistic plan that you can maintain for several weeks will always be more effective than trying to train more days than your body or schedule allows.

In the end, in any endurance sport, the athlete who is consistent in their training progresses and manages to rack up many weeks of training. The key here is not getting injured.

Burn this into your mind: the key isn’t just increasing miles, but correctly structuring training to improve progressively without putting the body at risk of injury.

How many miles a week should you run to prepare for a half marathon?

Weekly mileage volume is one of the most important factors in preparing for a half marathon. However, just like with training days, the appropriate and recommended amount depends, first of all, on the runner’s level and, logically, on the desired race goal.

In general, most 21K training plans move within these guideline ranges (we’re talking about approximate weekly kilometers):

• Beginner runner: 20-30 km weekly, to achieve the goal of finishing the half marathon feeling good.
• Intermediate runner: 30-45 km weekly, to achieve the goal of improving a personal best.
• Advanced runner: 45-60+ km weekly, when you have a competitive goal and seek to optimize performance.

For runners starting out in the distance, racking up between 20 and 30 kilometers a week is usually enough to develop the necessary endurance, always including a progressive long run that gradually approaches 18-20 km.

More experienced runners usually fall between 30 and 45 kilometers a week, which allows for the introduction of quality workouts like intervals, fartleks, or race-pace runs.

Above 45 kilometers a week, training is usually oriented toward more ambitious goals, with a higher workload and more specific sessions. This mileage volume is only suitable for runners with proven experience; trying to rack up this training volume without proper muscle and joint preparation is taking a serious risk of injury that shouldn’t be taken lightly.

The importance of progression

More important than the total number of miles is how volume is increased throughout the weeks of preparation. A common rule among coaches is not to increase weekly mileage by more than 10% compared to the previous week, which helps reduce injury risk and makes it easier for the body to adapt to training.

Additionally, it’s worth remembering that rest is also part of training. Alternating higher-load weeks with lighter weeks allows for better assimilation of the work done.

What is a “normal” time for a first half marathon?

It depends on the level and previous experience:

Runner Level	Estimated Half Marathon Time	Common Goal
Beginner	1h50 – 2h20	Finish
Intermediate	1h35 – 1h50	Improve time
Advanced	Sub 1h30	Performance

In a first experience, the main goal should be to finish feeling good and learn from the process.

Common mistakes when preparing for a half marathon

• Increasing mileage too quickly
• Not doing strength work
• Neglecting recovery
• Wearing new running shoes on race day
• Not practicing your nutrition/hydration strategy

Preparing for a half marathon (21.097 km) generally requires no less than 10-12 weeks (for runners who already run 10Ks regularly and comfortably) of progressive training, as well as balanced planning between load, recovery, and nutrition. However, many runners make mistakes that can affect performance on race day or even cause injuries during preparation.

These are some of the most common ones and how to avoid them.

Increasing mileage too quickly

One of the most common mistakes is trying to improve too fast. Many runners, especially those preparing for their first half marathon, increase their weekly mileage volume excessively.

As a general rule, it’s not recommended to increase weekly volume by more than 10% compared to the previous week. This progressive increase allows the body to gradually adapt to the stress of training.

When the increase is too sudden, the musculoskeletal system doesn’t have enough time to adapt, and the chances of suffering injuries like these increase:

• shin splints
• tendinopathies
• plantar fasciitis
• muscle strains

Additionally, it’s advisable to introduce deload weeks every 3 or 4 weeks, slightly reducing volume to favor recovery and consolidate training adaptations; this is exactly what sports planning is about, and that’s why it’s so important to put yourself in the hands of a coach who knows how to modulate loading and deloading phases.

Not doing strength work

Many runners focus all their preparation on racking up miles and forget about strength work. However, strength is a fundamental component of running performance.

A basic strength program helps to:

• Reduce injury risk
• Improve joint stability
• Optimize running economy
• Maintain technique when fatigue sets in
• Promote recovery between sessions

The most recommended exercises are those that involve large muscle groups and movement patterns similar to running, such as:

• Squats
• Lunges
• Deadlifts
• Core work (planks and glute bridges)
• Ankle and foot exercises

Ideally, you should perform between one and two strength sessions per week, especially during the early phases of the training plan.

Neglecting recovery

Training isn’t the only factor that determines performance improvement. Recovery is when the body adapts, strengthens, and supercompensation occurs, leading to performance gains.

Sleeping less than 7 hours a night can directly affect several important processes for the runner:

• muscle recovery
• hormonal regulation
• adaptation to training
• concentration and decision-making ability

Besides sleep, recovery also includes other important aspects:

• Rest days or easy training
• Proper nutrition
• Sufficient hydration
• Muscle release techniques (foam roller, massage, cryotherapy…)

Ignoring recovery usually results in accumulated fatigue, performance plateaus, and higher injury risk.

Wearing new gear, especially running shoes, on race day

While it might seem like a good idea to run in brand new running shoes (or sports gear in general) on competition day, wearing new gear on race day is a common mistake.

Running shoes must be previously tested in several workouts, especially in long runs. This allows you to check:

• if the fit is right
• if they cause chafing or pressure points
• how they respond at race paces
• if they’re comfortable during prolonged efforts

It’s best to use running shoes that have between 75 and 100 kilometers of use, so they’re already adapted to your foot but still have their cushioning properties intact.

Not practicing your nutrition/hydration strategy

Nutrition and hydration during the race must also be trained. A common mistake is trying gels or isotonic drinks for the first time on the day of the half marathon.

It’s very important to be clear that everyone tolerates energy products differently. Additionally, the digestive system needs to adapt to consuming carbohydrates during effort; this is what nutritionists call “training the digestive system”.

Therefore, it’s recommended to:

• Test energy gels during long runs
• Check the amount of water needed to ingest them correctly
• Identify/test which products sit best on your stomach

Additionally, in races longer than 90 minutes, it’s usually recommended to ingest between 30 and 60 grams of carbohydrates per hour, provided you’ve previously trained this strategy.

Practicing nutrition in training helps avoid digestive issues and allows you to arrive at the race with a clear and tested strategy.

Mental and pacing strategy for race day

The half marathon is best managed in three blocks:

Km 0–7: Control

• Start slightly below your target pace
• Avoid getting carried away by the initial adrenaline

Km 8–15: Stability

• Find a sustainable pace
• Control breathing and posture

Km 16–21: Effort management

• This is where the real challenge begins
• Mentally divide the distance into small blocks
• Focus on technique and cadence

Many runners lose time due to poor initial management rather than a lack of fitness.

Frequently Asked Questions about how to prepare for a half marathon (FAQ)

Is it possible to prepare for a half marathon in 8 weeks?

Yes, but with caveats: only if an aerobic base is already established; starting from zero isn’t recommended. An 8-week plan can be enough if the runner already has a consolidated aerobic base, meaning they’ve been training regularly for months and can run at least 8-10 kilometers comfortably.

If you’re starting from zero or after a long period of inactivity, it’s not recommended because 8 weeks are insufficient and significantly increase the risk of strain or injury. In those cases, it’s best to first spend several weeks building a solid endurance base before starting a specific plan to prepare for a half marathon.

The key isn’t just completing the distance, but doing it with physical confidence and enjoying the process.

How many miles a week should I do to prepare for a half marathon?

For an average recreational runner, the typical range is between 30 and 45 kilometers a week, spread over 3 to 5 sessions. This load allows for developing endurance, working on specific paces, and maintaining balanced progression.

However, the ideal volume depends on previous level, experience, and goal (finishing the race or seeking a personal best). More miles don’t always mean better results. In fact, excessive volume without proper recovery can lead to accumulated fatigue and increase injury risk.

Training quality—the right combination of well-planned long runs, controlled pace sessions, and complementary work—is just as important as the total number of miles.

Should I do 21 km runs before the race?

It’s not necessary to complete the official distance in training. In fact, most preparation plans set long runs of 18 or 19 kilometers as a maximum reference.

The goal of the long run is to stimulate aerobic endurance and get the body used to maintaining prolonged effort, not to arrive completely fatigued before competition day. Leaving a few kilometers “in reserve” allows you to reach the start line feeling better physically and mentally.

The race atmosphere, adrenaline, and pace management will do the rest on race day.

Is strength really necessary to prepare for a half marathon?

Absolutely yes. Strength training isn’t just a fundamental complement to preparing for a half marathon; it’s also a guarantee for success. It not only improves performance but also significantly reduces injury risk.

Working on strength—especially in the legs, core, and stabilizing area—improves running efficiency, technical economy, and the ability to maintain good posture when fatigue sets in.

Additionally, stronger muscles help better absorb the repeated impact of each stride, which is key in a distance like the half marathon.

Including 1 or 2 weekly sessions of strength in your preparation can make the difference in both performance and athletic health.

Is it better to train by feel or by pace?

It’s best to combine both strategies, as each offers different advantages within half marathon preparation.

Training by pace (min/km) allows you to objectify the work, control intensity, and adjust sessions to a specific goal, especially in quality workouts like intervals, tempo, or race-pace runs. It’s a useful tool for measuring progress and avoiding going too fast or too slow in key sessions.

However, training only by watching the clock can make you lose connection with your body’s signals. Factors like rest, temperature, stress, or terrain directly influence daily performance.

That’s why training by feel is equally important. Learning to identify perceived effort—easy run, controlled pace, high intensity—helps better regulate load, prevent overtraining, and adapt the plan when the body needs it.

In summary:

• Easy runs and long runs → prioritize feel.
• Specific workouts and target pace work → use pace references.

The balance between data and perceived effort is what allows for smarter and more sustainable long-term preparation.

Final tip: choose your first half marathon well

If you’re already thinking about preparing for a half marathon, let me give you one last tip for a good first experience: choose a well-organized race with an attractive course (without elevation or weather difficulties) and a motivating atmosphere (it’s not the same to run somewhere with crowds and entertainment as it is to do it in a desolate place).

See you at the starting line.

Health and miles.

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