Long run build up calculator

Long runs build endurance, but adding too much too soon raises injury risk. This spaces out the build week by week.

Your details

From 8.0 km to 16.0 km

10 weeks

up 10% a week with easier weeks built in

Week 18.8 km
Week 29.7 km
Week 310.6 km
Week 48.5 km
Week 511.7 km
Week 612.9 km
Week 714.2 km
Week 811.3 km
Week 915.6 km
Week 1016.0 km
Time on your feetadd how long your longest run takes now to see each week's time

The 10% rule is a sensible brake rather than a proven law. What studies do show is that big sudden jumps (over about 30% in a week) bring more injuries, so steady wins.

Pain that gets worse as you run, or that is there at rest or at night, is a reason to stop and see a GP or physio.

The working

8 km to 16 km

Rising 10% a week with every fourth week cut back to 80%: 8.8, 9.7, 10.6, then an easy 8.5; 11.7, 12.9, 14.2, then 11.3; 15.6 and finally 16 km. That is 10 weeks.

If your 8 km takes 50 minutes, the final 16 km takes about 1 hr 40 min, past the 75 minutes where carbohydrate on the run helps. With 12 weeks until your race, the build fits with 2 weeks to spare; with 8 weeks it is 2 weeks short, and a shorter longest run is kinder than rushing.

The 10% rule

Useful brake, not a law

A Dutch trial (Buist 2008) found the 10% rule on its own did not cut injuries in new runners. A Danish cohort (Nielsen 2014) found jumps over 30% in a week did. So: steady is good, sudden is risky, and easier weeks help.

Long run basics

How it should feel

Slower than your usual run: you can chat
Take water and, beyond 75 minutes, some carbohydrate
Walking breaks are allowed
The day after should be easy or rest

Read with care

What this cannot tell you

Your total weekKeep the other runs short while the long run grows.
Injury historyIf you get injured easily, use 5% instead.
PainStop if pain changes how you run.

Questions

Common questions

How much should I increase my long run each week?

About 10%, or 1 to 2 km, with an easier week every few weeks.

How long should my longest run be for a half marathon?

Most plans peak at 16 to 19 km.

Should every long run be longer than the last?

No. Cut back weeks are where your body catches up.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Before you push on

New to exercise, or got a heart, lung or joint condition? Have a word with your GP or practice nurse first. Stop and get help if you feel chest pain, faintness or unusual breathlessness, and call 999 if it is severe.