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 1977 formula (T2 = T1 × (D2/D1)^1.06) captures this with a single exponent validated against tens of millions of race results — the 1.06 power, not 1.0, is exactly the gap between "pace holds steady" and what human endurance actually does.
Estimate with the rule, then check it against the calculator that models it properly.
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