Hi
On your watch, open the Workout app and select to start an Outdoor Run.
When the goal screen is displayed, swipe left / right to the distance goal. This will be shown in KM. Force press on this screen, then choose miles.
Exit from the Workout app (go back one screen by pressing cancel, then press the Digital Crown).
The Activity app will now show miles - and miles will also be used for future workouts in the Workout app.