Running on a treadmill
Before every run, on your watch or your phone, pick Outdoor or Treadmill above Start. It starts on Outdoor each time, so pick Treadmill when you step on the belt. A free run on a rest or strength day offers the same choice.
Indoors there is no GPS, so your Apple Watch estimates your distance and pace from your stride, the way Apple's own Indoor Run does. It learns your stride on outdoor runs, so the estimate is close but drifts from the treadmill's own number. A treadmill run on the phone alone keeps time, and for a planned run works out the distance from each step's target speed.
When you end the run, Why Not shows Treadmill distance. Turn the Digital Crown on the watch, or type on the phone, until it matches the distance on the treadmill's display, then Save. That number is the run's distance everywhere: the summary, Activities, the run's detail, your week, your plan and Apple Health. Your pace and splits follow it.
Every target shows the belt speed beside the pace, in mph or km/h by your units, and the coach says it at each step, for example: Set the belt to 6.7. Pace alerts are off on a treadmill, because you set the belt. On a hill day the reps become incline: 6 percent for each rep and back to 1 percent for the recovery, and the coach calls each change.
Treadmill runs count toward your miles, your week and your plan, but they never set personal records, race estimates or your race fitness.