Further to the use of exercise everyone on a prolonged fast, this is what Dr Joel Fuhrman MD has to say on the matter............

"I give my fasters gentle stretching exercises to do to keep their backs flexible, and encourage them to walk around a little, but discourage any strenuous exercise.
Occasionally fasters develop some lower back discomfort. At first I attributed this to spending so much time in bed. But because this occasional occurrence of lower back discomfort resolves as the person continues the fast, I realized it was likely related to a biochemical rather than to a structural problem. Many aches and pains that appear and then resolve during the fast illustrate the heightened detoxification and reparative activity made possible by the fasting state.
Many people require less sleep during the fast than they do normally. One should still rest sufficiently and relax at night in a quiet room. Rest in the dark if you are unable to sleep, and do not fret or be concerned if you don't sleep much when you are fasting. Each person is different; try to rest and sleep as much as you can, but if you can't sleep much, that is OK, too.
We want to give the body a complete rest so that it can devote its energies to healing. Arguments, business-problems, and family problems should be avoided. This is a time for recuperation and the person should be resting and at peace physically and mentally. The faster should spend most of his or her time in bed either relaxing, sleeping, or doing gentle activities such as reading, writing, or listening to the radio".
in FASTING and EATING FOR HEALTH Page 218.

The view of Dr Alan Goldhamer of TrueNorth....................

WHY EXERCISE IS NOT A GOOD IDEA WHEN WATER FASTING
http://renegadehealth.com/blog/why-exercise-is-not-a-good-idea-when-fasting