The Riegel formula
Proposed by Peter Riegel in 1977, it links times over two distances: T2 = T1 × (D2 / D1)^1.06. The 1.06 exponent captures the natural slowdown as distance grows. It is the model Paceo uses to track your progress.
What are the limits?
The prediction assumes you are as well prepared for the target distance as for your reference race. From 10K to marathon, that is rarely true without long runs: most runners finish a few minutes over the predicted time. The closer the two distances, the more reliable the estimate.
Choosing a reference race
Use a recent race (under 2 months), run all out, on a flat, measured course. A half marathon is an excellent basis for predicting a marathon.