The Anaerobic Threshold Test
The 5K (3.1 miles or 5,000 meters) is a uniquely painful race. Unlike a marathon where you conserve energy, a 5K requires you to redline your engine almost immediately. You are running right on the absolute edge of your VO2 Max, filling your bloodstream with lactic acid, and forcing your body to clear it just fast enough to avoid collapsing.
Because of its short duration, predicting a 5K time requires looking at your speed-based performances.
Predicting the Sprint
If you have recently run a 1-mile time trial on a track, or if you are stepping down in distance from a recent 10K, you can accurately predict your 5K potential using Riegel's endurance decay formula.
The Formula
The Importance of the Warm-up
Because the 5K is run at such a high intensity, your body does not have time to "warm up" during the race itself (like it does in a marathon). If you start a 5K cold, your heart rate will spike violently in the first half-mile, causing you to hyperventilate and dramatically underperform your predicted time. A massive 15-minute dynamic warm-up is absolutely essential to hitting your calculator's predicted pace.