If you follow the steps I mentioned, you choose your activity app (run, cycle, swim, etc), NOT the workouts app. Ex. If you want to change data screens for a run workout, choose run and adjust your data screens there.
Workouts is a "repository app", not an activity itself. When you start a workout from there you then have to select an activity, Select the screen setups you want on the activities you will select (such as Run ,etc). You cannot change the fields on the front page that is managing the workout (so to say) that is dependant on the workout setup (Pace , Heart rate etc)
