Why Race Predictors Are Often Wrong
Your Garmin says 3:35 marathon. You run 3:48. A sharp 5K makes COROS and the PaceWOD Race Predictor look optimistic for the half. That gap is normal. Predictors estimate what recent data could support under favorable conditions — not a guaranteed finish time.
What the Models Actually See
Watches blend estimated VO2 max, recent races, training load, and HR–pace relationships. Formula tools (Riegel: T₂ = T₁ × (D₂/D₁)^1.06) extrapolate one result to another. VO2 sets a ceiling; threshold, economy, durability, and execution decide the race.
Worked Stretch: 20:30 5K → Marathon
Recent 5K 20:30. Raw Riegel-style marathon projection often lands near ~3:18–3:22 depending on implementation. Athlete’s longest run is 18 km, weekly volume 45 km, no race-pace fueling practice, forecast race dew point 18°C. Practical A-goal after haircut: add ~5–8% for short-race stretch + heat → closer to 3:30–3:35 pacing, with a B-goal slower still. Prefer anchoring off a half when possible. Compare tools on Pace Chart and VO2 Max Calculator; watch VO2 caveats: Garmin vs lab.
Do not treat a marathon prediction from a mid-build 5K as a contract. Models miss weather, hills, and whether you can actually take 30–60 g carbs/hour. Watch predictors use more training data than plain Riegel — they still miss race-day fueling and heat.
Why Longer Predictions Miss More
- 5K→10K is usually reasonable; 5K→marathon assumes durability you may not have
- Mileage and long-run specificity matter more than a single PR
- Hot dew-point races often cost minutes vs cool flat predictions
Fueling and Environment Gaps
Models usually assume carbs are available and weather is friendly. Rough glycogen stores cover roughly 90–120 minutes hard — not a full marathon at goal pace without 30–60 g carbs/hour (up to ~90 g/hr if gut-trained). Hot dew-point races often cost 5–15 minutes versus cool flat predictions.
Decision Rule
If the predicted distance is more than about 2× your recent race distance (or your long runs do not support it), add conservatism or re-anchor to a closer distance before setting goal pace. If training matches the distance and conditions are cool/flat, start near the predicted range and adjust mid-race from feel and splits — not from the watch’s single optimistic number.