🏃 Race Pace Predictor & Training Paces

Enter one recent race you actually ran, honestly, and this predicts the rest, gives you the pace to hold per kilometre or mile, and turns it into the five training paces you actually need. Predictions are only as good as the race you put in — a parkrun you walked part of will predict a marathon you cannot run.

Uses Riegel's formula, T₂ = T₁ × (D₂/D₁)1.06, fitted to world-class performances across distances and still the standard for predicting one race from another.
When to trust this, and when not to

Riegel's exponent of 1.06 was fitted to records from 200 m to the marathon. It works well over one or two steps of distance. Predicting a marathon from a 5 km time stretches it a long way, and the usual failure is that it is too optimistic — endurance does not scale as neatly as speed. Treat anything more than about four times your input distance as an upper bound on your ability, not a target.

Training paces are derived from the predictions, roughly on Daniels' model: easy running at about 1.3 times your 10 km pace, threshold at half-marathon to 15 km pace, intervals at 5 km pace, repetitions faster still. The single biggest mistake in amateur training is running the easy days too hard, which is exactly why the easy pace here will feel embarrassingly slow.