To assess readiness for a sub-three-hour marathon, the community suggests moving away from short-distance times (5K) in favor of a strong half-marathon performance (ideally under 1h25) combined with high and consistent weekly training volume.

noli's read

The community broadly agrees that a 5K time is an insufficient speed indicator to predict the endurance required for 42 km. Empirical evidence in sports physiology, such as that discussed in Jack Daniels' 'Running Formula', confirms that the correlation between distances decreases as the gap widens, making the half-marathon a much better predictor. The need for significant weekly volume (60-90 km/week) is supported by observational studies showing that the ability to accumulate mileage reduces the risk of performance decline at the end of the race. The idea that 'durability' or physical resistance is the limiting factor is a classic observation among progressing runners. While the consensus is strong on the importance of the half-marathon and volume, there remains an element of exaggeration in the notion that a specific time (such as 1h25) is an absolute cutoff, as individual physiology varies. The community is converging toward an approach based on the accumulation of evidence in stages rather than a single test.

the sources

1. 1r/AdvancedRunning