Hi, I'm exercising during my fast, but if I feel like I'm going to faint, then I don't exercise. It depends a lot on how you feel, today I felt really pumped up and energetic and did 15 min. on the treadmill and another 45 min. on the ellyptical.
That said, there's no point in feeling guilty for not exercising during a fast. After all, the goal is to detox and a lot of rest may be needed. Plus most of us have jobs/kids/school, so we're up and about anyways, it's not like we're sleeping all day.
My personal goal is that ideally during a fast I am able to exercise and weight train, just to keep my metabolism from slowing down too much. Plus if I weight train, I feel better knowing that the weight loss I am seeing on the scale is really fat and not muscle (which weighs more than fat, and burns fat).