Steve

I too was hitting the gym 5x a week for weightlifting, elliptical, as well as yoga and pilates classes. Like the old Cure song: "I had the best laid plans this side of America" to keep up a schedule similar to yours, but you know what they say about the best laid plans. The first week, no problem. The second week, a little weary. The third week, vomiting after class. The fourth week thru eighth week of radiation: couch potato.
The only downside is that the more you exercise, the more calories you will need and as Christine noted, that can quickly get problematic. I made it thru without a PEG the first time in part because I had so much lean muscle mass going into TX. I think you are ahead of the game by being in very good shape.
Keep the Faith
Charm