Treadmill workout tracking on apple watch

I have found that treadmills are very bad at this type of monitoring.

Make sure the your personal information is up to date in the health App.

Height, weight, age, sex

Reset your fitness calibration data

From your iPhone>Watch>Privacy>Reset Fitness Calibration Data>Tap this

You will just get a confirmation box that tells you that your metrics will not be as good if you do this.

Tap confirm and do it. You will get nothing else, no confirmation but it is done.

Then go for a 20 minute outdoor run or walk using the workout app at your normal speed on a flat road/path etc

This will re-calibrate the watch

I too have found Apple watches are terrible at tracking both pace and distance on treadmills. I just finished a 6 mile run at a 7:10 pace on the treadmill, however, my watch said 5.5 miles with an 8:00 minute pace. Just stick with what the treadmill said. If possible, get outside and do the outdoor run as that one is accurate.