As IdrisSeabright suggested, choose a workout closest to the rest of your circuit, or, if it consists completely of planks (which I think would be unusual) and nothing else you do in a workout choose Other.
While there is a long list of possible workouts, whichever you choose isn’t that important except to show the breakdown of how you exercised in the Activities app. For all workouts the same basic factors go into what is reported, primarily duration and heart rate (used for intensity) and is converted to calories. Where appropriate steps are also recorded separately, but steps are continuously recorded, independently of a specific workout.
For example, if you choose a treadmill or elliptical workout it will record the steps in that workout (but also in the daily total), but if you choose weight lifting it will only report your steps around the gym.