A 25-minute 5K predicts almost exactly a 52-minute 10K, not a 50-minute one — endurance pace slows down as distance increases, not just because a race is longer but because fatigue compounds.
Peter Riegel’s formula (T2 = T1 × (D2/D1)^1.06, 1977 and 1981), fitted to world-record performances, captures this with a single exponent — the 1.06 power, not 1.0, is the gap between "pace holds steady" and what human endurance actually does. For recreational runners it holds well up to the half marathon (Vickers and Vertosick, 2016, 2,303 runners).
The exponent is fitted to trained runners over common race distances. Extrapolate from a 5K to a marathon and it is optimistic for anyone who has not done the endurance training — the wall at 30 km is a fuelling limit the formula knows nothing about.
At 2× the rule says 2× and the exact answer is 2.085× — 4.1% low. It holds to within 5% up to 2.4×, then drifts.
The "plus a bit more" is real and it compounds: about 4% at double the distance and 8% at four times, which is the difference between a good day and a bad one over a marathon.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
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