How to go from a 10K to a half marathon: These are the keys to do it with guarantees

If you are at that moment when you feel that you have already mastered the 10 km, you have completed several races of that distance or you have been moving in that range for months, maybe you are considering taking the leap to the half marathon, but yes, it is true, it can be imposing. It is normal that you feel doubts, but also that you are curious to venture into those 21.097 km. The change is remarkable: it is not only twice the distance, it is a challenge that requires greater aerobic endurance, better pace management and a proper nutritional strategy.

But don't worry, I'm going to try to help you approach this transition with a plan that offers you security, based on scientific evidence and real sports practice. This article is designed for you who want to make the leap and will need a clear roadmap to train your body (and your mind) towards that goal. Prepare your running shoes, we're getting started...

Why is the half marathon different?

More time in the aerobic zone

On a physiological level, most runners complete a 10K in 60 minutes. The half marathon, on the other hand, exceeds an hour and a half for the average half marathon runner. According to several studies in the International Journal of Sports Physiology and Performance, the ability to maintain a pace close to your lactate threshold for more than 70-80 minutes becomes decisive. In other words, it is not enough to "suffer hard" as in a 10K; you need to tolerate a somewhat lower intensity but sustain it twice as long.

Fuel strategy

In a 10K, you might not worry too much about drinking or ingesting gels. In a half marathon, training fat oxidation and carbohydrate utilization becomes more relevant. Muscle glycogen can be depleted if you haven't prepared your body for longer runs or if you don't carry some supplies.

Initial assessment: are you ready to take the leap?

Your current level

• Can you complete 10K with relative ease at light paces?
• Have you been running regularly (3 days per week or more) for at least 6-8 months?
• Have you experienced at least a couple 10K races and know your average pace?

Health check

It is important to rule out injuries and have a medical green light, especially since your mileage volume is going to increase by 20-30% during the preparation phase.

Baseline test

Do a 12-14 km run at a very easy pace: see if you end up very tired or if your muscles suffer from fatigue. This "mini-test" will show you to what extent your body can tolerate longer distances.

Phase 1: Building a solid base (4-6 weeks)

Objective: To progressively raise your aerobic capacity and get your joints, tendons and muscles used to the increased mileage. The key word here is progression.

Tasks we will incorporate

Easy runs (between 30 and 60 minutes): Around 75-80% of your maximum cardiac frequency or at a pace where you can carry out a full conversation without choking.

Strength training: According to research from the Journal of Strength & Conditioning Research, 2 weekly strength sessions (squats, lunges, core work...) improve running economy and reduce risk of injury.

Basic technique and speed: Add short sprints (8-12 seconds) on a hill or flat at the end of a run (2-3 reps). This generates power and does not load the system too much.

Volume increase

A 10% weekly or every 2 weeks in your total mileage. If, for example, you were running 35 km/week after your 10K, increase to 38-39km the first week and maintain for a couple of weeks to assimilate.

Phase 2: Targeted training (6-8 weeks)

Here we begin training specifically geared to the half marathon. After the base, we introduce key sessions:

Lactate threshold or "threshold runs".

Fundamental to sustain paces close to your half target.

Studies from the American College of Sports Medicine (ACSM) show that training at 85-90% of your maximum heart rate (or 60 minute race pace) improves resistance to lactic fatigue.

Example: 20-25 minutes at "uncomfortable but steady" pace. Progress to 30-35 minutes in a row or split into 2x20 with 2 gentle minutes between sets.

Increasing Long Run

This is the centerpiece of the plan. Go from rolling 12-14 km to 18-20 km gradually. Add a set at "marathon" or "half marathon" pace (e.g., last 20 min at a pace 10-15 sec/km slower than your goal).

Evidence suggests that long runs should not exceed 30% of total weekly volume to avoid overtraining.

Controlled intervals

Although the half marathon is run slower than a 10K, you should not forget some spark. Suggestion: 6-8 reps of 800m or 1km, at a pace slightly faster than your target pace, resting 90 sec.

Again, the key is regularity: don't aim to "kill" on each repetition, but maintain a steady pace.

Strength and stability training

Maintain 2 days per week of 20-25 min of strength exercises ( free weights or self-loads) focusing on quads, hamstrings, glutes and core. The half marathon requires a strong musculature.

Phase 3: Polishing and taper (final 2 weeks)

Volume reduction

Maintain intensity but cut 20-30% in total mileage the second to last week and 40-50% the week before the race.

Make sure you don't put in any runs longer than 60 min in that last week.

Key session

7 days before, a 6-8km test run at half target pace. It will give you confidence and fine tune sensations.

Control your hydration, carbohydrate intake and rest. Good sleep on those days is as important as training.

Slight spark in the race

On Tuesday or Wednesday before the race, run a gentle 30 min + 5-6 20 second accelerations to remember the speed stimulus without fatiguing yourself.

Daily plan (simplified example of 8-10 weeks)

Keep in mind that we are not starting from 0. You have already been training for months, you have already participated in several 10k races.... If so, you can continue reading:

Weeks 1-2

Monday: Rest or very light running (30 min)

Tuesday: Running 45-50 min + 3 hill sprints

Wednesday: Strength in gym (squats, lunges, core)

Thursday: Threshold run 2x10 min (2 min light in between)

Friday: Rest

Saturday: Progressive running (40-50 min)

Sunday: Long run 12-14 km

Weeks 3-6

• Add volume to long run (up to 16-18 km).
• Threshold run goes to 3x10 min or 1x25-30 min.
• Intervals: 6-8x800 m at pace 10-15 sec/km faster than target.
• Maintain strength training 2 days/week.

Weeks 7-8

• Long run reaches 18-20 km.
• Threshold: 30-35 min straight at lactate pace.
• Adjust rest if you feel overloaded.

Week 9 (Taper)

• Reduce volume (20-30% less).
• A final test of 6-8 km at race pace 7 days before the race.

Last week: Light running (30-40 min) +1-2 days of total rest.

Nutritional advice

Duration > 75 min: Consider 60 g/h of carbohydrates (gels, bars or drinks) in long workouts of more than 90 min.

Experiment in training: Don't try anything new on race day. Test gels or isotonic drinks during your 16+ km runs.

Avoid common pitfalls

Gaining volume too fast: Risk of injury (fascia, periostitis, etc.). Keep the 10% progression.

Always train at the same pace: The body stagnates, alternate easy days and threshold or interval days.

Forgetting strength: Legs need robustness to withstand 21 km.

Not resting enough: Needless to say, without recovery, there is no adaptation.

Your path to 21K

Making the leap from 10K to the half marathon is an exciting and achievable challenge if you plan smartly. Build a solid base with easy runs, introduce threshold training and progressive long runs, and don't forget about strength and recovery. By relying on scientific evidence and the experience of thousands of runners, you can prepare your body to tackle those 21 kilometers with greater confidence. There are no shortcuts: the key is consistency and gradual adaptation. If you take care of every step of the way, the half marathon will be a rewarding adventure and, who knows, maybe just the prelude to a full marathon. See you at the finish line!