Mark,
Give your body and cycling a little credit - it kept it out of your lymph nodes, which in turn let you avoid chemo with the radiation.

Regardless of your prognosis, it's a challenge to stay positive and not cloud your days with the shadows of the future. You've got some fatigue going from the radiation treatment as well as the trauma of the fact of the cancer itself: both of these will take time to resolve. In getting back to his (horseback) riding, my husband would push himself a little more each time, and get more rest and sleep than he thought reasonable. Like Eric says, its exercise AND rest.

I like Charm's description of sidestepping bad thoughts very much. Some of the meditation techniques might prove useful.