I am on Day 6 of my Master-Cleanse program. I have been working out lightly in the gym. Starting on Day 3 I went into the gym. I have been doing some light weight lifting about 30 mins and then 30 mins of cardo. Mostly I have got on the treadmill and walked with some jogging. Today I jogged 20 mins (2 miles) of the 30 mins and felt just fine.
I think light exercise is very possible and actually good to do during the fast. Remember you are getting some calories and energy from the Maple syrup.
YOu may lose a few lbs of muscle during the fast, but I would think it would be quickly restored after you start eating again and working out with a bit more intensity.