How I Estimate Your Fitness and Predict Your Race Times

VO2max — maximum oxygen uptake — is the single best predictor of endurance performance. It measures how efficiently your body uses oxygen at peak effort. Elite marathoners are around 70-85 ml/kg/min. Recreational runners are typically between 35-50. I estimate yours from your personal bests using the Jack Daniels VDOT formula. A 5K in 25 minutes maps to a VDOT of roughly 38. A 5K in 20 minutes is about 49. The math uses velocity-based oxygen cost curves and exponential decay functions for the effort percentage at different durations. It is complicated. But the input is simple: your best race time at a known distance.

If I know your 5K time, I can predict your 10K, half marathon, and marathon times. I use Riegel's model: predicted time = known time * (new distance / known distance) ^ 1.06. The 1.06 exponent is the key. It means performance does not scale linearly with distance. It degrades. Double the distance takes more than double the time, because fatigue accumulates non-linearly. A 20-minute 5K runner will not run a 40-minute 10K. They will run closer to 41:30. The model is accurate for trained runners. It is less accurate for beginners, who usually slow down more than the formula predicts, and for ultramarathon distances, where nutrition, psychology, and gut tolerance matter more than VO2max. For 5K through marathon, Riegel's 1.06 has held up for over 40 years.

Beyond raw fitness, I track consistency: how regularly you are training over a rolling 4-week window. The score runs from 0 to 100. High variance, such as training hard one week and disappearing the next, scores low. Steady, regular sessions score high. This matters because consistency is the strongest predictor of improvement. A runner who does four easy runs every week will outperform someone who does two hard sessions every other week. I also use your consistency to calibrate my recommendations. If you have been inconsistent recently, I dial back the intensity. If you have been steady for a month, I push harder.

Weekly average pace is a noisy metric. One interval session can make the whole week look faster, and one recovery run can make it look slower. So I use 4-week rolling averages and report trends as percentage change. The leaderboard shows these trends too. A slower average pace is not a bad sign. It might mean more easy runs, which is often exactly what a runner should be doing. Pace trends are context. They show where you have been, not where you are headed.