Long Runs: How Long and Why

The long run is the one session most runners protect. It asks the body for something the shorter runs do not: to keep producing energy for a long time, mostly from fat, while the legs absorb thousands of repeated impacts. That request drives a specific set of changes.

What a long run builds

Muscle stores carbohydrate as glycogen, and at a steady effort those stores run low after roughly 90 minutes to 2 hours. A long run goes there on purpose. The body responds by storing more glycogen next time, building more mitochondria in the working fibers, and burning more fat at a given pace so the glycogen lasts longer. More capillaries grow around the fibers, improving oxygen delivery. These are the adaptations that raise the pace you can hold for a long time, which is what decides most long races.

Tendons, bones and the small stabilizing muscles get a dose of durability a 40 minute run cannot give. Those tissues adapt over weeks, which is why the long run causes the most trouble when its length jumps too quickly.

There is a mental side too. Running on heavy legs while the brain asks to stop is a skill, and the long run is where it gets practiced.

How long is long enough

For most people, the long run should be roughly 20 to 30 percent of the week's total mileage. At 30 miles a week that is a long run around 8 miles; at 50, near 12 to 15. Past about 2.5 to 3 hours, most coaches judge that the added fatigue and injury risk outweigh the added adaptation for anyone not training for an ultra, so marathon runners usually cap it there, short of race distance.

Grow it slowly. A mile or so every week or two, with a shorter week every third or fourth, keeps the tendons in step with the lungs.

Pace matters as much as distance. Most of the long run should sit below your first ventilatory threshold, the point where breathing first picks up, which keeps the fat burning adaptation in play. That heart rate comes from a test, not a formula, which is why we build zones from a person's own thresholds.

Fueling on the run

Under about 75 minutes, water is enough. Past that, roughly 30 to 60 grams of carbohydrate an hour, started early rather than once you fade, keeps the pace steady in the last third. Well trained runners can go higher, up to around 90 grams an hour, by mixing sugar types. Practice it on training runs, because the gut adapts to fuel on the move just as the legs adapt to the miles.

A long run pulls together pace, fuel and the heart rate zones that make it work. Those zones should come from your own numbers. That's what a VO2 test gives you.

This article is educational and is not medical advice.

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