Race predictions and Critical Speed

Runsei predicts your race times from your own best efforts, not a generic formula. Across hard efforts, the distance you can cover rises almost linearly with time: distance = D' + Critical Speed × time. Critical Speed is the slope — the fastest pace you can hold in a true metabolic steady state, roughly your best *sustained* effort for about 30–40 minutes. Go faster and fatigue accelerates sharply; stay under it and you can hold on. D' is a small "anaerobic reserve" of distance you can spend above it. This is the critical-power model from exercise physiology (Monod & Scherrer; Jones, Vanhatalo & Poole), applied to running.

Critical Speed is close to — but not the same as — your threshold pace. It sits a touch *faster* than the lactate-threshold pace your tempo/threshold sessions are prescribed at (physiologically it's the boundary just above your maximal steady state). So Runsei uses Critical Speed to model races and shape your zones, and prescribes your actual threshold work from your measured threshold pace — the two numbers are related, not interchangeable.

Once Runsei knows your Critical Speed, it predicts any race: time = (distance − D') ÷ Critical Speed.

Honesty is built in:

• Only genuinely hard efforts at different durations are used — easy runs are ignored, and Runsei keeps only your best pace at each duration.
• Each prediction carries a confidence, and predictions outside the range of efforts you've actually done are flagged as extrapolation. The marathon is always an extrapolation — the model doesn't know about fuelling and late-race fatigue, so treat long-race numbers as a ceiling, not a promise.
• If you haven't done hard efforts across a range of distances, Runsei says so rather than inventing a time. A couple of best efforts — say a hard 10 minutes and a hard 30 minutes — sharpen it a lot.