I got my Apple Watch today, and I found that the Workout app was displaying my distance in miles. I think in kilometers, and I’ve changed the Health app to display kilometers, which is an easy-to-find setting. On the Apple Watch, however, it is hidden.
Go to the Workouts app. Select a workout, and swipe to the screen that shows distance.
Force tap (tap and hold until you feel taptic feedback), and you’ll see the following screen:
Tap MI or KM, according to the unit you want to use.