How it works
Riegel's formula: T2 = T1 x (D2 / D1)^1.06. Riegel's 1981 paper found fatigue factors between about 1.05 and 1.08 for running from 1.5 km to the marathon; we use 1.06, which sits inside the range Riegel reported. Predictions assume you have trained properly for the longer distance.
Source: Peter S. Riegel, "Athletic Records and Human Endurance," American Scientist 69(3):285-290 (1981).