I too will be starting a fast Monday. I want to do a 40 day fast. I want to start with a water fast, then add juice if I can't handle it. I've done three fasts since Dec, and it is still hard for me to have energy just on water. I hope to clean more toxins out this time, so I will have more energy.
I hope someone with experience with exercising on a fast will post to answer your question. I did a little exercising last fast, but only on the first day and then on the days I had juice. I wonder if since you are already use to exercising a lot, that you have little problems exercising while fasting.