How to train for a half marathon: weeks, long runs and pacing

Most runners who can comfortably run about 5 km three times a week can train for a half marathon in 10 to 16 weeks. The training is built around one long run that grows gradually towards 16 to 20 km, a few easy runs, an easier week every so often, and a short taper before the race. The running plan generator builds that structure from your start date and current longest run, and this guide explains the reasoning behind each part.

Are you ready to start?

A half marathon is 21.1 km, or 13.1 miles. Before starting a dedicated plan it helps to be running regularly for a couple of months and to manage around 30 minutes, or 5 km, without stopping. If you are not there yet, finish a beginner plan first; Couch to 5K: how the nine-week plan works and how to finish it covers that. Coming from zero straight into half-marathon training squeezes too much increase into too few weeks, and that is when shins, knees and Achilles tendons complain. If you have a health condition or have not exercised in a long time, check with a doctor first.

How many weeks to train

Twelve weeks suits most people who already run regularly. Ten is workable if your long run is already 8 to 10 km; sixteen or more gives a newer runner room to repeat weeks. The running plan generator accepts 10 to 24 weeks for a half marathon and spreads the build evenly across whatever you choose. Longer is not automatically better: a plan that drags on is harder to stick to, so choose the shortest length that lets the long run grow gently from where it is now.

What a training week looks like

Three to five runs a week is typical. Most of them should be easy, at a pace where you could hold a conversation. Easy running builds the aerobic engine, strengthens tissues and lets you recover for the sessions that matter. In the plans the generator makes:

• Easy runs fill most days, at a relaxed, conversational pace.
• One run with strides for beginners: an easy run finished with four to six relaxed 20-second accelerations, which keep your legs used to a quicker rhythm without hard training.
• One quality session for intermediate runners, alternating intervals (such as 4 × 1.6 km at half-marathon pace) with tempo runs at a comfortably hard effort.
• The long run at the weekend, again at easy pace.

If you use heart rate, the easy days belong in the lower zones described in Heart rate zones explained: how they're calculated and what each one trains; the heart rate zones calculator works out your numbers.

Building the long run

The long run is the core of half-marathon training. It teaches your body to run for well over an hour and gives you confidence on race day. The generator starts from your current longest run and builds steadily towards a peak of about 19 km (12 miles), reached shortly before the taper. Every fourth week is a cutback week with a long run about a quarter shorter, which gives your body a chance to absorb the training. You do not need to run the full 21.1 km before race day; the taper and race-day adrenaline cover the last few kilometres. Run long runs slowly, often a minute or more per kilometre slower than your goal race pace. A common guideline is to avoid raising weekly distance by more than about 10 percent at a time; the exact figure has little research behind it, but the principle of gradual increases is sound.

Choosing a race pace

The best guide to a realistic finish time is a recent race or time trial at a shorter distance. The race time predictor uses Riegel's formula to project a 5K or 10K time to the half marathon, and the pace calculator turns a target time into a pace per kilometre or mile with splits. Predictions assume you have trained for the longer distance, so treat them as an upper limit if your long runs have been short. Practise your goal pace in the later weeks, in short sections within easy runs or in tempo sessions, so that it feels familiar.

Taper, fuel and race day

In the final one to two weeks, reduce your mileage so you arrive rested. The generator's half-marathon plans cut the last long run to about three quarters of the peak, then make race week very light, with an easy shakeout run before the race. Keep a few short faster sections so your legs stay sharp; The marathon taper: what to do in the last three weeks covers the reasoning. Practise fuelling on long runs: if you expect to be out for more than about 90 minutes, try energy gels or a sports drink in training, never for the first time on race day. On the day itself, start slower than you feel you could, settle into your pace after the first few kilometres, drink to thirst, and save any push for the final third.