Why shorter races can over-promise
Prediction equations extend a known race result to a longer distance. That works best when the effort was recent and representative, and when training is appropriate for the target event. A fast 5K does not prove that the legs, fueling plan and pacing discipline are ready for a marathon.
The missing variables
- Weekly volume and long-run consistency
- Heat, wind, hills and course congestion
- Fueling, hydration, illness and sleep
- Whether the input race was truly maximal
Use the range for planning
Build pacing options around a range, including a conservative plan. Update the estimate after relevant races and long training blocks. The output predicts a time under assumptions; it does not promise a result or determine whether entering an event is medically appropriate.