I have also become athletic in fifties. I am into running. Running does have an issue with overloading of joints but it can be managed by cross-training couple of times a week. Cycling, I find too risky, especially on roads. With all these mobile phone obsessed drivers out there, I just don't feel comfortable. To me, an athlete is who does about one hour of physical activity per day. There are quite a menu of choices available with walking, swimming, running, strength training, yoga, gardening, cycling, sports like tennis, squash, ...
