Belt-appropriate work
Controlled intervals (exact paces, exact recoveries), bad-weather tempo runs, easy recovery mileage with zero navigation load, and treadmill-specific hill work (incline intervals). The belt's precision is a feature for workouts where pace discipline matters more than terrain realism.
Road-necessary work
Long runs (fueling practice, terrain variance, boredom tolerance), pacing practice (the skill of effort-vs-pace on real ground), and anything specific to your goal race's surface and hills. The belt cannot teach downhill eccentric strength or wind management — race day will, expensively, if the road miles were skipped.
The calibration bridge
Once a month, run the same workout on both surfaces and compare heart rate and feel at claimed-identical paces. Most runners discover their belt reads 2–5% optimistic — data that recalibrates every indoor session afterward. The goal is not treadmill-vs-road purity; it is knowing the exchange rate.