I believe you can go into Workout, go to workout you want "reset", tap three-dots, tap on Open. This will start a new workout, so do this when about to start that type of workout. Stop as usual and should be "reset" to initial values and will be the default when using that workout type again.
I only use open for all my workouts so that's the first option in my list of workouts under specific workout types, so a little bit of a guess on my part.