How long to train for a marathon? A plain-English answer

Why 16 to 20 weeks is the usual answer

A marathon is 42.2 km (26.2 miles). You don't need years of racing behind you to finish one, but you do need a runway long enough to build your long run up safely and then ease off before race day. That runway is what coaches call a training block: a set number of weeks with a clear start, a middle where the work gets harder, and an end where you rest up.

Sixteen to twenty weeks is the common range because it fits three things in without rushing any of them: a few weeks of base running, a build of roughly ten to fourteen weeks where the long run grows, and a taper — the final two to three weeks where you cut the distance right back so your legs arrive fresh. Cram all of that into eight weeks and the build is too steep.

In practice, the number matters less than the shape. A calm 18-week block beats a frantic 12-week one every time.

What decides how long you need

Two runners can both enter the same race and need very different runways. The things that change the answer:

• What you can run now. If you can chat through a 60–90 minute run, 16–18 weeks is realistic. If 5 km (3.1 miles) still feels long, you need base-building months first.
• How often you can run. Three or four runs a week is plenty for a first marathon. More days means more room to build, but life has to allow it.
• How long you've been running consistently. A few steady months behind you makes a big difference to how well you absorb the training.
• Your injury history. If something has flared up before, build more slowly and get it looked at by a GP or physio before you start.
• Everything else. Sleep, work, small children, a house move. A block that fits your real week is the one you'll finish.

If you're starting from nothing

Then the honest answer is longer, and that's fine. A common path is: learn to run 5 km (3.1 miles) comfortably, spend a few months running regularly, build to 10 km (6.2 miles), then a half marathon, then start a marathon block. That's often a year or more from your first run to the start line — and it's the version that tends to be enjoyable rather than exhausting.

The beginner's first month is the right place to begin. You don't need to sign up for anything yet.

One useful anchor: the NHS suggests adults aim for at least 150 minutes of moderate activity a week, and running counts. That's a good weekly target while you're building, long before any marathon talk.

What the weeks actually contain

Whatever the total length, most marathon blocks follow the same rhythm:

• Base weeks. Easy running, gradually more of it. Most of your training should be at a pace where you can hold a conversation.
• Build weeks. The long run gets longer, a little at a time, and one session a week gets harder — often intervals or a tempo run (a sustained comfortably hard effort).
• Peak weeks. Your longest runs, usually somewhere around 30–35 km (18.6–21.7 miles), done as time on feet rather than a race.
• Taper. Two to three easier weeks. It feels strange. Trust it.
• Throughout. Easy runs, one or two strength sessions of 20–40 minutes, and at least one full rest day. Strength work is what keeps hips, glutes and calves together late in a long run.

Practise drinking and eating on the long runs exactly as you plan to on race day — a gel or two can turn a miserable last hour into a manageable one. There's more on that in what to eat before a run.