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Your first half marathon: how to train for 21.1 km without getting hurt

How long to train, what a good week looks like, why strength work earns its place, and how to taper and fuel so race day goes to plan.

16 October 2026 · 8 min read

Runners in a sunny road race on a street lined with palm trees — Photo: Capstone Events on Unsplash

The half marathon sits in a sweet spot. It is long enough to feel like a real achievement, and short enough that you can train for it alongside work, family and everything else. Most people who can run 5 km today can be ready for 21.1 km in about three months.

The hard part is rarely the distance itself. It is getting to the start line healthy. Roughly a third of recreational runners pick up a running injury over a year and a half, and most of those are overuse injuries: the body asked to do more than it was ready for.^[1]

This guide covers what the research says about building up safely, and how to arrive at race day fresh.

How long do you need?

If you can already run for about 30 minutes without stopping, plan on 10 to 12 weeks. That is the length most beginner plans settle on, and it gives you room for a few easy weeks when life gets in the way.

If 30 minutes of running is still a stretch, start earlier. Spend four to eight weeks building to it first, using run-walk intervals if you need them. There is nothing second-rate about walking breaks. In a randomised study of 42 recreational marathoners, the group that took regular walking breaks the whole way finished in a similar time to the group that ran non-stop, and reported less muscle pain and fatigue afterwards.^[2]

Coming back after a break, or returning from an injury? Give yourself the longer runway. Fitness comes back quicker than your tendons and bones adapt, so the plan should follow the slower of the two.

What a good training week looks like

Three or four runs a week is plenty for a first half marathon. Each run has a job.

A runner jogging alone along a quiet road at sunrise — Most of your week should feel this calm. Photo: Tikkho Maciel on Unsplash

Easy runs. Most of your running should feel comfortable enough to hold a conversation. This is where your heart, muscles and tendons do most of their adapting, and it costs little recovery.
One long run. This is the backbone of the plan. It starts around 6 to 8 km and builds to 16 to 19 km a few weeks before race day. You don't need to run the full 21.1 km in training; race-day adrenaline covers the last stretch.
One quicker session (optional). Once the easy running feels settled, add one session with some effort in it: a few 5-minute blocks at a pace you could hold for an hour, or hill repeats. Keep it to one a week.
Two short strength sessions. More on these below. They can sit on easy-run days or rest days.
At least one full rest day.

Why so much easy running? When researchers compared recreational runners on an easy-heavy plan (about 80% of time at low intensity) with runners spending far more time at moderate effort, both groups improved their 10 km time over 10 weeks. The easy-heavy group improved a little more (5.0% against 3.6%), though the gap was small enough to be chance.^[3] Mostly easy running gets you there and leaves energy for the sessions that count.

A week in the middle of a 12-week plan
Monday	Rest
Tuesday	Easy run, 30 to 40 min, plus strength
Wednesday	Quicker session: warm up, 3 × 5 min steady-hard, cool down
Thursday	Rest or a short walk
Friday	Easy run, 30 min, plus strength
Saturday	Rest
Sunday	Long run, 12 to 14 km, easy pace

How fast to build up

You have probably heard of the 10% rule: never add more than 10% to your weekly distance. It is a reasonable instinct, but the evidence for it is thin. When 532 new runners were split between a cautious 10%-rule plan and a standard plan, about one in five got injured in each group.^[4]

Newer data points somewhere more useful. A 2025 study followed more than 5,200 runners through their GPS watches for 18 months. Injury risk didn't track weekly totals so much as single runs. When one run was more than 10% longer than the longest run in the previous 30 days, the risk of an overuse injury went up, and bigger jumps carried more risk.^[1]

In practice: grow your long run steadily, roughly a kilometre or two at a time, and never let one session leap far past anything you have done recently. Every third or fourth week, cut back by about a quarter so your body can catch up.

Strength work: the part most plans skip

Many half marathon plans are a list of runs and nothing else. That leaves out one of the best-supported things a runner can do.

A woman doing a goblet squat with a kettlebell in a gym — Two short strength sessions a week go a long way. Photo: Meagan Stone on Unsplash

Two large reviews of the research found that strength training makes distance runners faster. Heavier lifting, close to the most you can manage for a few reps, helped running economy (how much energy each stride costs) and time-trial results more than lighter loads did. Combining heavy lifting with jumping drills did best of all.^{[5], [6]}

The evidence that strength work prevents injuries outright is less clear. What it does do is make the tissues that take the load stronger. Your calf muscles alone handle several times your body weight on every stride, thousands of times per run.^[7] Stronger calves, hips and thighs have more margin before something complains.

Two sessions of 20 to 30 minutes a week is enough. A good starting set:

Calf raises, straight and bent knee, building to single-leg with added weight
Split squats or lunges, for the thighs and hips, one leg at a time
Romanian deadlifts or hip hinges, for the hamstrings and glutes
Step-ups onto a box or stair
A side plank or hip abduction, for the muscles that keep your pelvis level

Start light enough to learn the movements, then make them genuinely hard: by the last couple of reps of each set you should be working. Ease the load back in the final week before the race.

Staying on the right side of sore

Some aches come with training. The skill is telling the normal ones from the ones that are trying to tell you something.

A useful rule of thumb: mild discomfort, around 3 out of 10, that eases as you warm up and has settled by the next morning is usually fine to train through. Pain that sharpens as you run, changes the way you move, or is worse each time you go out is a signal to stop, swap the run for something gentler, and get it looked at if it doesn't settle.

The usual trouble spots for half marathon runners are the shins, the Achilles tendon, the knees and the outside of the hip. Almost all of them flare up after a jump in load, which is why the build-up rules above matter more than any piece of kit.

A few habits make the training stick:

Sleep. It is when your body turns training into fitness. If a busy week costs you sleep, cut a run rather than the night.
Eat enough. Under-fuelling a growing training load is a common, quiet cause of fatigue and stress injuries.
Treat the plan as a guide, not a contract. Missing a run is fine. Squeezing two into one day to catch up is not.

The last two weeks: taper

Your fitness for race day is built by about two weeks out. What is left is to arrive fresh. A large review of tapering studies found the best results came from a taper of about two weeks in which training volume dropped by roughly 40 to 60%, while the number of sessions and their intensity stayed the same.^[8]

So keep running on your usual days, keep a few short efforts at race pace, and make everything shorter. Many runners feel sluggish or restless during a taper. That is normal and doesn't mean you are losing fitness.

Fuel and race day

For a run of one to two hours, the research-based guidance is up to about 30 g of carbohydrate per hour; past two hours, closer to 60 g.^[9] In practice that means a gel or a few sweets around halfway if you will finish in under two hours, and one every 30 to 40 minutes if you will be out longer. A normal, carbohydrate-rich dinner the night before and a familiar breakfast two to three hours before the start are enough. A full carb-load is for longer races.

A runner taking a cup of water from a volunteer at a race aid station — Practise drinking and fuelling on your long runs, not for the first time on race day. Photo: Leviosa Hou on Unsplash

The golden rule: nothing new on race day. Practise your breakfast, gels and drinks on long runs so your stomach knows what to expect.

Finally, pacing. The most common race-day mistake is starting too fast on fresh legs and adrenaline. Aim for an even pace, hold the first few kilometres slightly slower than feels natural, and save any push for the last 5 km. Passing people at the end is far more fun than being passed.

A plan built around you

Everything above is a starting point, not a prescription. The right plan for you depends on where you are starting, how many days you can train, what equipment you have, and whether an old knee or tendon needs a gentler build. A downloaded 12-week PDF, or a running app that hands everyone the same schedule, can't know any of that.

That is what Achilles is for. Pick Finish an endurance event as your goal, choose the half marathon, and answer a short set of questions about your running, your week, your kit and anything that bothers you. Achilles then builds your plan from a library of sports-science research: the runs, the strength sessions and the exercises that suit you, each with its sets, reps, rest and how to progress, and the studies behind it.

As you log your sessions, the plan moves forward with you, so the long run grows at a pace your body can absorb and the strength work keeps up.

See how Achilles works