I think an intensive half hour, or two pops in in an hour, with a WG will keep you fitter and leaner than time spent in a gym (provided of course you don't gorge yourself afterwards).
Punting is certainly more enjoyable than a gym, as a sex workout will burn up a lot more calories than any other form of workout.
I'm surprised that WG's don't extol the weight loss inherent in punting as part of their advertising.