How long should your long run be?

The long run is the session most runners are proudest of and most likely to overdo. It’s the backbone of endurance — the run that teaches your body to keep going — but the idea that a longer long run is always a better long run is how people arrive at their goal race already worn down. Distance is a dose. The right dose depends on the goal.

Roughly how far, by goal

These are ballparks for the peak long run in a block, not week one:

• 5K: you barely need a “long” run in the marathon sense. Something in the 8–12 km range, easy, is plenty of aerobic support.
• 10K: 12–16 km. Enough to build endurance without stealing from your quality days.
• Half marathon: 16–20 km. You don’t need to run the full race distance in training — getting close is enough.
• Marathon: 30–32 km is the common peak. Notably, that’s not the full 42.2. Running the whole distance in training costs more recovery than it gives back for most amateurs; the race-day adrenaline, taper, and fuelling cover the gap.

Build it like a staircase, not a ramp

The long run should grow gradually — a rough guide is no more than about 10% longer than your recent longest, and not every single week. A useful pattern is to step up for two or three weeks, then pull the long run back for a recovery week before climbing again. That zig-zag lets your body consolidate the gains instead of just absorbing damage. A long run that jumps from 16 km to 26 km because the calendar said so is an injury with a nice view.

Effort matters as much as distance

Most long runs should be run easy — conversational, controlled, finishing tired but not hollowed out. The point is time on your feet and aerobic development, not proving anything about pace. Marathoners layer in some segments at goal pace as the race nears, but the default long run is an easy run that happens to be long. If you’re wrecked for three days afterward, it was too long, too fast, or both. (Same principle as easy runs generally.)

When a long run should be cut short

Here’s the part fixed plans get wrong. The long run is the most demanding session of the week, which makes it the most sensitive to how you’re actually doing. Show up to a 30 km run on four hours’ sleep and a resting heart rate five beats high, and forcing the full distance doesn’t build fitness — it digs a hole you’ll spend the next week climbing out of. The right move is often to trim it: run 20 of the 30, bank real work, protect the block. Trimming a long run is not failing it. Deleting it would be — so the honest adjustment is to shorten, not skip.

That judgement is exactly what Faster makes for you. It builds the long run ladder toward your race, then reads each morning and protects the session when your body can absorb it — and trims it, rather than cancels it, when it can’t. The mileage that matters is the mileage you recover from. The goal stays 42.2. The road there bends around the runner you actually are.

Training guidance, not medical advice. Big jumps in long-run distance are a common injury cause — build gradually, and see a professional if pain persists.