I avoid exercise when I water fast. I try to walk, but usually Water Fasting is very physically taxing.
What kind of fast are you starting? I'm also considering a juice fast because a Water Fast IS so intense and I do want to exercise. But we'll see.
(Just out of curiosity, have you found that your metabolism gets wonky after a fast? I tend to GAIN way more weight following a fast, it's like my body hoards every single calorie)
I've decided to postphone my fast until either Monday or Tuesday because I have a ton of food events to go to and plan and I don't want to have to explain...last time I had to "justify" my lack of eating, and it was not a very supportive environment.