I been using a GPS watch's (GARMIN) and I decided to bought an apple watch,
has been a long discussing with apple support experts regarding calibration process but so far this watch is totally inaccurate,
This is not a real GPS, Apple sale it as GPS but is NOT a GPS,
GPS has a global correction, this watch has a .02 error per mile, so if you run 10 miles the error will be 0.2 mile finally you will get 10.2 miles total and this will affect of course your pace.
if you run with your phone, that could reduce the error, but once you leave you phone at home, you will have the error again.
My bad, for NOT made a research before to buy it,
Hope this experience help to someone.