Edit distance on Apple Watch?

I find treadmills to be very bad at this.

In the gym I go to there are 20 of them and none of them match.

They don't even have the same time calculations.

My watch is the same every time no matter which of the 20 I use

If you think it is your watch try re-calibrating

iPhone>My Watch>Privacy>Reset Fitness Calibration Data

To add to this, the reason why things like distance and pace are off is because of the different ways the different devices track you. Apple Watch uses things like arm movement and GPS to track steps and distance, whereas a treadmill uses speed to calculate distance. This is also why you see a difference in things like calories- the Watch has an always going heart rate sensor whereas the treadmill does not. It’s just an unavoidable problem unless you find a gym that has GymKit enabled devices. Hope this helps!

No - it is not possible to edit a workout after it has been saved.