It's easy to have your Apple Watch show you all of your activity details in miles or kilometers, whichever you prefer —except that Apple doesn't make it obvious how to do it.
Lets Dig in
This is one of those Apple Watch settings that has to be done on your iPhone. And it's one of those Apple Watch settings that you do not control in the iPhone's Apple Watch app.
Instead, open the Health app. Then:
Tap on Activity
Scroll down to Walking + Running Distance and tap
Scroll down to Unit and tap
Choose between km and mi for kilometers and miles
If you do this with your Apple Watch's Activity screen open and scrolled down to your Total Distance today, you'll see the units change instantly.