I like your reasoning with the exercise afterwards, makes sense to me. I do think there is something to it. The last time I did a 'fasting' type diet, I afterwards incorporated brisk walking 3-4 kilometers daily (something I hadn't done before the cleanse). It really helped to keep me pumped, exhilarated, happy and I didn't gain much more than 5lbs. In addition to that though, I kept on a mostly alkaline diet to keep my body in good working condition, and had smaller portions. So no sugar, dairy, wheat, etc.
Now though I'm starting over again because those nasty bad habits have crept back, looking forward to getting back in the swing of things again!