ARC

Fitness is the engine you've built — a rolling average of roughly your last six weeks of running. It moves slowly in both directions, which is exactly why one missed run barely dents it. Sports science calls this CTL — Chronic Training Load; same number your watch app may show.

Fatigue is the load you're carrying from just the last week or so. It spikes fast after hard days and fades fast with rest. Science name: ATL — Acute Training Load.

Form is the difference between them — how race-ready you'd feel today. Science name: TSB — Training Stress Balance (fitness minus fatigue). Here's the counterintuitive part: negative form during hard training is normal and good — it means you're absorbing work and building. You only want it positive when it counts.

Is my number low or high? Neither — it's yours. This scale is deliberately not 0–100, because running fitness has no ceiling: someone just starting sits around 8–15, a regular runner 25–45, a serious amateur 50–70, elites past 100. Comparing your number to anyone else's is meaningless — the only references that matter are the three marked for you: where you started, where this plan peaks, and (if you've set a time goal) the line your goal needs you to cross. Progress is the slope, not the level.

The horizontal axis is your plan timeline in weeks; the vertical scale is training-load points — the single currency the whole engine runs on (a typical easy run is worth ~30–50 points of daily load, and your fitness number is your sustainable daily average). Solid lines are what's actually happened; dashed lines are where the current plan takes you. When training toward a race, the green zone at the finish is where the taper is designed to land you: fit and fresh at the same time — which is the whole trick of peaking.