Why is there a workout icon / runner icon / fitness app icon on my watch face?

When the runner status icon is shown on your watch face, it indicates that your watch presently has a workout in progress:

You may have a workout paused or in progress in the Workout app or in a third-party fitness app such as Strava.

If you have been using Strava or any other third-party app, you will need to end the activity via that app on your watch (not on your iPhone).

This user tip was generated from the following discussion: Why is a little fitness app icon on my watch face now.