In my mid-60s I don't have the legs/lungs to ride fast anymore, and that's okay. On my fitness bike I average 12-13, 14-15 on my carbon road bike. I want to get back into longer rides but don't know if I have the stamina to train for centuries, so I'm cutting myself off at half-century (ok, and maybe metric century). I've got several half-centuries coming up and I need to train to ride faster, because for some reason I think it's bad that a half-century will take me 4 hours I guess it's interval time, though I loathe doing 'em.