How do I overcome all-or-nothing thinking about exercise with cancer?

I am a 77-year old competitive athlete diagnosed 3 years ago with high-risk localized prostate cancer and treated with radiation and androgen deprivation two years ago. I seem to be quite different from the cancer patients referred to in the accompanying article, because it never occurred to me to do anything but keep doing all of my regular exercise every day, regardless of the many debilitating side effects of my treatment. Quite the contrary: it’s competitive athletics that has been my refuge and saving grace in battling cancer. True, the treatment has robbed me of the stamina needed to keep running marathons, so I downshifted to specializing in the 800m and 1500m distances in track. And I’m no longer nationally ranked in any competition. But if I didn’t have athletics, I doubt I’d be alive today.