Sub-3 marathon training: should you run a 100-mile week?

Running a 100 mile week has a certain mystique in marathon training. It sounds obsessive, even reckless, but for sub-3 runners it can also be transformative. The real question is not whether 160km is too much in theory, but whether you can absorb it in practice and still recover well enough to improve.

I averaged over 100 miles for seven straight weeks, peaking at 120 miles at age 45. This wasn’t a one-off stunt but a deliberate block where I seized a window of opportunity, factored in recovery, and kept listening to my body. I came out the other side stronger, not broken, and ran a PB in London despite hot conditions. Done right, big weeks provide a foundation of aerobic depth and leg toughness that no shortcut can match. The marathon rewards durability, and the final 10K is where you find out whether your legs have had enough time on the road.

That said, 100 mile weeks aren’t essential, and they come with risks. Injury, burnout, and constant fatigue are all real dangers if you rush the build or ignore the warning signs. The esteemed sports scientist Tim Noakes, in his classic text Lore of Running, suggests that around 120km per week is the sweet spot for most distance runners, warning that beyond this you often hit diminishing returns. I respect that view. But I also think the ceiling is personal: the right mileage is the most you can run, recover from, and sustain while still getting fitter. For some, that means holding 100 miles for a few weeks as a plateau; for others, it may never be realistic.

It’s also about when you do it. Hitting 100 miles the week before your marathon is a recipe for disaster — the benefit comes when you build big weeks in the middle of a cycle and then allow your body to freshen up. The taper is what converts all that volume into race-day sharpness. And it’s not only about neat calendar weeks either: think in rolling seven-day totals, because the stress of a 30K Sunday run doesn’t vanish when the clock strikes midnight. Planning in real time, not just diary weeks, helps you avoid accidental overload.

The key is not to treat the round number as a badge of honour. A 160km week is only valuable if it helps you adapt. That means guarding recovery as fiercely as training, keeping most miles easy, and being honest about whether you’re progressing or just grinding yourself down. It also means being pragmatic: if life opens up a gap where such mileage is possible, seize it; if it doesn’t, then protect consistency at a lower ceiling and don’t look back.

Mileage is still the biggest performance driver in marathon training. If you can run a 100 mile week and bounce, it will give you an edge. If you can’t, then the highest sustainable mileage you can repeat week after week is what will get you to sub-3. The marathon is a test of who turns up with legs ready to go again, not who can log the biggest week on paper.