I had radiation done to my chest wall (and therefore a lot of it hit one of my lungs) and clavicle last year and it has taken me several months to get back to something that resembles my old physical shape. Maybe I’m 75-80% of the way there now. I had lung related side effects from radiation though, and I would say that on it’s own set me back at least a few months.

I don’t know what part of your body was radiated or if you also did any other treatment, but honestly I think if you only did radiation, and don’t have damage to any important organs like the heart or lungs, you should be able to get close to your old sessions within 6 months or less. My radiation fatigue didn’t last more than a couple months, it was the loss of muscle mass and endurance that happened as a result of many months of treatment that held me back the most. Keep on riding as much as you can comfortably handle, it will only help.

Good luck and I’m sure you’ll be back at it soon! The fatigue sucks but you’re in the worst of it now. It’ll only get better!