Predict Your Marathon Time From a Half marathon

How a Half marathon predicts your Marathon

This is the prediction runners trust most, and for good reason. A half marathon is long enough that your endurance is already in the picture, so the jump to the full is mostly about pacing and fueling rather than raw fitness. If you have raced a half in the last month or two, you hold the single best predictor of your marathon there is.

The math lines up neatly here. The marathon is exactly twice the half, and Riegel multiplies your half time by about 2.08. That is close to the old coaching rule of doubling your half and adding ten to twenty minutes. The slower and less marathon-specific your training, the closer to the twenty-minute end you should plan.

Doubling your half, and the wall the formula cannot see

Doubling a 1:40 half gives 3:20, but the honest prediction is closer to 3:28, and many runners finish slower still. The reason is the wall, the point around 20 miles where glycogen runs low and pace becomes a battle of will and fueling. Riegel scales your half cleanly, but it has no term for the specific fatigue of the marathon’s final hour.

That is why the back half of your training matters more than the prediction. Long runs of eighteen to twenty-two miles, practiced race-day fueling, and a disciplined early pace are what turn a half based prediction into a real result. Bank time in the first ten miles and the wall will take it back with interest. Start a few seconds slower than predicted and let the second half come to you.

Want every distance at once? The race pace predictor shows your time for the whole range and prints a pace band, and the VDOT calculator turns this race into your training paces.