I was about 50 pounds overweight.
I still mowed the lawn, chased after my 2 year old and climbed stairs.
I had fluctuations in my energy but I didn't push it by working out since I hadn't been doing that for a long time before I started.
I have been doing Body Flex (deep breathing and holding stretches) for about a week now. It has really moved alot inside me and it's not too strenuous.
If you have been working out before starting the fast, I would suggest slowing it down a little. This is not the time to test the "No Pain, No Gain" theory.
Your insides (muscles, digestive system) are not getting what they are used to (food) so they might rebel by cramping, pulling, etc.
It is important to move (at least walk) to help your cells push out the toxins.
Good luck.