Hi
For the most accurate results, be sure to record specific fitness activities as workouts using the Workout app (or a third-party app) and to choose the activity type that most closely matches your workout, including choosing Other for workouts that are neither included within nor sufficiently similar to those in the main list.
This enables Apple Watch to use the most appropriate sensors and data sources for that activity type when tracking results.
When recording treadmill-based walks or runs, recorded via the Workout app and selecting Indoor Walk or Indoor Run as the activity type, the Workout app tracks your arm motion (measured by the accelerometer) when estimating results including distance and pace:
Use the Workout app on your Apple Watch - Apple Support
For the best results, allow the arm on which you are wearing your Apple Watch to swing naturally as you move around and during treadmill-based workouts.
Calibrating your Apple Watch can improve the accuracy of activity-related estimations. Calibration teaches your watch how your arm movements relate to your stride lengths at different speeds when you are walking and/or running. It does this - during outdoor walks and/or outdoor runs recorded via the Workout app - by comparing data from the accelerometer with GPS / Location Services data.
More information and instructions for calibrating your Apple Watch is available via the link below. The article also includes instructions for resetting any existing calibration data and starting afresh. Resetting your calibration data will not erase your Activity history:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Activity-related estimations also depend, in part, on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
More information:
Use the Activity app on your Apple Watch - Apple Support