Predicting race times with the Riegel formula
If you know how fast you can run one distance, can you predict how fast you'd run another? Pete Riegel, an engineer and long-time runner, tackled exactly this question in 1977 and published a formula that's remained a standard reference in distance running ever since:
T2 = T1 × (D2 ÷ D1)1.06
The exponent of 1.06, rather than 1.0, captures something runners know intuitively: pace naturally slows as distance increases, even for well-trained athletes. A pure linear scaling (exponent of 1.0) would assume you could hold exactly the same pace from a 5K all the way to a marathon, which doesn't match real-world endurance physiology.
Where the formula works best
Riegel's formula tends to be most reliable when predicting a target distance reasonably close to the known distance, and for runners with a solid aerobic training base behind them. Predicting a marathon time purely from a 5K result introduces more uncertainty, since marathon performance depends heavily on long-run-specific endurance and fueling strategy that a short race doesn't test.
Using the prediction practically
Treat these numbers as a training target, not a guarantee. If your predicted marathon time looks faster than feels realistic given your actual long-run experience, that's useful information too — it may mean dedicating more training time specifically to endurance and fueling before race day, rather than assuming raw speed alone will carry you through the full distance.