It's not feasible to see the Workout duration on the Apple Watch while performing certain exercises. Whether it's a static hold, or an exercise that is interval based, I end up using the stopwatch feature of the Clock app on my iPhone to easily reference time intervals during workouts.
At the start of each workout, I have to manually sync the iPhone Stopwatch and the Workout Duration. While not the biggest deal, it is another step in the process, and if I pause the watchOS Workout, I then have to stop the Stopwatch in iOS at the same time, and then re-sync when I resume the Workout.
Adding a feature in the Activity section of the Apple Watch iOS app would allow users to sync the Workout duration to their iOS Stopwatch section. Pausing or ending a Workout stops the iOS Stopwatch.
Additionally, when adding a new workout, this split will be added instantly in the iOS Stopwatch so a user can directly see how long each workout has been going, without having to end the Workout and go into the Fitness iOS app.