Mark

If you browse thru DavidCPA's posts, you will see that he is also a dedicated cyclist who bikes the mega miles like you.
After 5 years, his cycling has not let his cancer "slip through" and he's a great example of a complete recovery.
My suggestion on how to overcome those thoughts is that it's more like aikido or judo instead of karate. You just cannot destroy or put down the thoughts/fears of the cancer coming back. instead you just sidestep them and gently but forcibly guide them to pass harmlessly by, without landing a blow. Acknowledge them but don't dwell on them. Eric's scientific facts may help you realize that exercise helps your recovery and your immune system.
Congratulations on getting thru TX.
Keep the faith
Charm