Sam,
Each of us and our recovery is different, for sure...you need to listen to your own body but I also know it's helpful to learn about other people's benchmarks. I'm not young, not old (58) and was active before GK (June 2006) and continue to be so. I was and am a runner, hiker, biker and skier. Four days after I flew back from my GK treatment, I climbed a 13,800 foot peak here in Colorado with my husband. I was definitely tired and definitely exhilirated!!! For about 2 months after the GK treatment, I tired more easily and required more rest but none of this prevented me from doing the active things I wanted to do. Some of this for me, I think, was mind over matter and some of it was being fortunate that my body was not seriously adversely affected. I count myself lucky in so many ways and hope to continue down that path. My very best wishes to you as you sort this all out. When the time is right you will know it...I think that pushing yourself can be a good thing if that's your mind set. Take care and good luck!
Ginny