In my case, I found it was important to keep moving, but not to push. If I pushed too hard with exercise (especially early on), I'd be in bed for a week.

For me, consistency matters more than intensity: I tried to get in the habit of exercising every day, even if I couldn't manage more than a short morning walk sometimes on a low-energy day.

*Definitely* don't get into the exercise-as-punishment mindset, because your body's already taking a beating from the cancer; think of moderate exercise as self-care instead.