Treat the prediction as a scenario with assumptions
A running race-time predictor extends one performance to another distance using a mathematical relationship. It does not observe your durability, course, weather, health, pacing, or fueling. The output is therefore a planning range to test, not a promised finish time.
Worked example: a 50-minute 10K
The model uses target time = known time × (target distance ÷ known distance)^exponent. With 50 minutes for 10 km and exponent 1.06, the half-marathon output is about 1:50:19 and the marathon output about 3:50:01. These are arithmetic projections, not evidence that the runner has trained for either distance.
A larger exponent predicts more slowing as distance increases. Do not tune it simply to obtain a desired goal; compare assumptions with recent representative performances and distance-specific preparation.
Decide whether the input race is comparable
The Riegel model was developed from performance records and assumes that speed declines with distance. The exponent is not a personal physiological constant. Change it only when you understand why, and show several scenarios rather than tuning it until it produces the desired answer.
Build a prediction range
Run the calculator with a conservative, central, and stretch assumption. Compare those results with recent long runs, pacing stability, weekly consistency, and race-specific practice. For a marathon, a strong half marathon cannot substitute for late-run durability or fueling. For a 100K, a road result cannot encode trail surface, climbing, darkness, or aid-station time.
Use the marathon pacing guide, half-marathon pacing plan, and ultra cutoff planner to convert the scenario into event-specific decisions.
Safety boundary
Do not use a predicted time to race through chest pain, fainting, severe breathing difficulty, illness, or pain that changes normal movement. This calculator is educational and not medical advice, readiness clearance, or a guarantee.