I'm in my seventies, and recognise this problem. Following the advice above would undoubtedly help . . . as long as you can tolerate gyms/pumping iron etc., which doesn't appeal much to me. I try to ride in the hills regularly, which is specifically helpful for cycling, of course, increasing intensity and resistance.
Depends where you live. Pretty hilly round here (W.Yorks), and I'm spending next week in the N.York Moors. Will only be cycling for an hour or two most days, but wrestling my way up one 20 percenter after another should do something for my hill-climbing. Not sure whether it will increase muscle-mass, but the likes of Froome etc seem to manage OK without it