How long should my long run be?

For a marathon, 18 to 22 miles at peak, capped at about three hours. For a half, 11 to 14. For a 10K, 10 to 12. As a rule the long run sits at roughly 25 to 30 percent of weekly volume.

What pace should the long run be?

Easy, roughly 60 to 90 seconds per mile slower than marathon pace, unless the session deliberately includes goal-pace segments.

Should I run 20 miles before a marathon?

One to three runs of 18 to 20 is standard, with the last about three weeks out. If that would take you over three hours, cap it at time instead. There is no evidence that 22 beats 18 for a first marathon and good reason to think it costs more.

Can I split a long run into two runs?

It is better than nothing but not equivalent. The specific adaptations come from continuous duration. Splitting is a reasonable fallback when the schedule genuinely does not allow the full run.

Guide

The long run, properly

The long run is the most important session in endurance training and the one runners are most superstitious about. Most of the superstition is about distance, when the variable that matters is time on your feet.

What it is for

The long run develops the things that only respond to duration: fat oxidation at a given pace, glycogen storage, capillary density, and the structural resilience to keep running well when tired.

It is also the only session that rehearses the specific problem of a long race, which is not running fast but running when your legs have stopped cooperating. That skill is trainable and it is trained here.

How long, by race distance

Peak long run as a share of a typical week, and in practical terms.

Race	Peak long run	Roughly
5K	8 to 10 miles	60 to 80 minutes
10K	10 to 12 miles	75 to 95 minutes
Half marathon	11 to 14 miles	90 to 120 minutes
Marathon	18 to 22 miles	2:30 to 3:00

How fast

Mostly easy. For a marathon runner that is roughly 60 to 90 seconds per mile slower than goal marathon pace. The long run is not a test and running it hard every week is a reliable way to arrive at race day flat.

The exception is deliberate quality inside it. From the middle of a marathon block, finishing a long run with marathon-pace miles is one of the most valuable sessions available, precisely because it rehearses running at target pace on legs that are already tired. Build from 4 miles at goal pace inside a 14 mile run toward 10 to 14 at goal pace inside an 18 to 20 mile run.

That version is a hard session and should be counted as one of your two quality days for the week, not added on top of them.

How to progress it

Three up, one down. Extend for three weeks, then cut back by a quarter to a third. 13, 15, 17, 13, 16, 18, 20, 15. The down weeks are when the adaptation consolidates.
Add roughly a mile a week. Or ten percent, whichever is smaller. Jumping from 13 to 17 because you felt good is the classic route to a stress injury three weeks later.
Practise fuelling on every one over 90 minutes. Race day is not the place to find out how your stomach handles a gel. Use exactly what you plan to use in the race.
Keep the day after easy. Or off. The long run is only worth what you can absorb from it.

Frequently asked

How long should my long run be?: For a marathon, 18 to 22 miles at peak, capped at about three hours. For a half, 11 to 14. For a 10K, 10 to 12. As a rule the long run sits at roughly 25 to 30 percent of weekly volume.
What pace should the long run be?: Easy, roughly 60 to 90 seconds per mile slower than marathon pace, unless the session deliberately includes goal-pace segments.
Should I run 20 miles before a marathon?: One to three runs of 18 to 20 is standard, with the last about three weeks out. If that would take you over three hours, cap it at time instead. There is no evidence that 22 beats 18 for a first marathon and good reason to think it costs more.
Can I split a long run into two runs?: It is better than nothing but not equivalent. The specific adaptations come from continuous duration. Splitting is a reasonable fallback when the schedule genuinely does not allow the full run.

Stop reading. Start training.

This is the theory. Shed is the schedule.

Knowing what a threshold run is for does not tell you which Tuesday to run one, or how fast, or what to do when you miss it. Shed writes every session of your race block and adjusts it as you train. Ask it where a number came from and it will tell you.

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