How long do I need to train for a marathon (or 5K, 10K, half)?

How the calculator works

How long you need to train depends less on the event than on where you start. The calculator looks at your longest run in the last month and works out how many weeks it takes to build that safely to what the event needs, with recovery weeks and a taper. It is one of the free LeCoach running calculators.

The rules are simple:

• Your long run grows by 10 % a week, or by at least 1 km, whichever is bigger.
• Every 4th week is easier, so you absorb the work. In practice, one recovery week for every three weeks of growth.
• A taper of 1 week for shorter events, or 2 weeks for events with a minimum build of 12 weeks or more.
• Never below the event's minimum build, even if you already cover the distance. Fitness is more than one long run.

How long should I train for a 5K?

Most runners need 6 to 10 weeks to prepare for a 5K. If your longest recent run is short, say 3 to 5 km, plan for the longer end of that range so your long run can build to around 8 km without rushing it. If you are already running 8 km or more comfortably, you have covered the distance: a 6-week sharpening block of faster intervals and tempo work is enough to bring your time down.

How long does it take to train for a 10K?

Typically 8 to 12 weeks. The long run needs to reach about 14 km. From a recent longest run of 8 to 10 km, that takes just a few weeks of growth, so the 8-week minimum usually applies. Starting from 5 km or less, expect to need the full 12 weeks to build safely. If you already run 14 km or more regularly, you have covered the distance and a 6-week build focused on pace will serve you better than more distance.

How long do I need to train for a half marathon?

Usually 10 to 14 weeks. The long run needs to reach about 18 km. From a recent longest run of 15 km, that takes only a couple of weeks of growth, so the 10-week minimum applies. If your longest run is closer to 8 or 10 km, lean toward 14 weeks so the long run can grow gradually without a spike in injury risk. Runners who already cover 18 km or more have the endurance in place; the remaining weeks are about getting faster at that distance.

How many weeks does it take to train for a marathon?

Most runners need 16 to 20 weeks. The calculator never goes below 16, because a marathon needs a long run of about 30 km and enough weeks around it to adapt. From a recent longest run of 15 km, 16 weeks is enough; from 10 km it takes 18 weeks, and from 5 km about 24. If your longest run already reaches 30 km or more, you have covered the distance, and a focused 6-week build aimed at marathon pace and fuelling will do more for your time than extra long runs.

When you already cover the distance

If your longest recent run already matches or beats what the event needs, the calculator still recommends a minimum build, because racing well takes more than one long run: you need pace work, recovery and a taper too. In that case the result reads: "You've covered the distance. To improve your time we recommend at least a 6-week build." If your event is sooner than that, you will see a gentler note instead of a warning, because finishing is well within reach even if a full build is not.

Why the long run sets the timeline

Endurance events are limited by how long you can keep going, so the long run is the clearest marker of readiness. It is also where most people get it wrong: jumping from 15 to 25 km in a fortnight is the classic way to get injured or burnt out before race day. Growing it in small steps lets your muscles, tendons and energy systems adapt together.

The rest of the week matters too. Your total volume should grow alongside the long run, and the weekly training volume planner shows how to build it week by week with the same recovery rhythm. The phases of a full plan, from base to taper, are explained in base, build, peak, taper.

What if you have less time?

If your event is closer than the calculator says, and you do not already cover the distance, you have three honest options:

• Pick a later event. The best choice if you want to race rather than finish.
• Aim to finish rather than race. Build what you can at the same safe rate, keep the taper, and pace the day conservatively.
• Choose a shorter distance. A half marathon instead of a marathon, or a 10K instead of a half.

What you should not do is skip recovery weeks or the taper to squeeze in more growth. That trades a small gain in distance for a big risk of arriving tired or injured.

Common mistakes

• Entering your best-ever long run instead of the last month's. What you did last year does not count now.
• Counting the event distance as the target. You do not need to cover the full distance in training.
• Building the long run but not the rest of the week. One long run a week on top of very little else is a fragile base.
• Skipping the taper. The last one to two weeks are when the fitness shows up.

When the numbers change

• Starting from zero: if you have not been running at all, start with a few weeks of regular easy running before the build counts.
• Coming back after illness or a break: rebuild first. Our guide to training after illness or time off shows how.
• Experienced runners with years of training may need fewer weeks of growth, but the event minimum still applies.
• Injury or niggles during the build mean pausing growth for a week, which adds a week to the plan.

Tell LeCoach your event date and it builds the whole plan back from it: base, build, peak and taper, with every run written.