It gets harder as you get older I’m 50 and still try to do what I did when I was 25 and I can’t, I’m slowly giving up trying to and now just go and enjoy myself, if you push too far it takes ages to recover I have found, but I still push myself because its fun to do so, just not to the point of blowing up and injuring myself. If I enter a duathlon ,xc race etc I don’t go in to win ( I don’t train enough or have the lightest bike or the the best trainers ), I do it for me and to just get the best performance I can,with what I have, and most of all to have fun. :D