I'm 76.  Anything over 20 miles with a few hills and I need a day or two off the bike.  I lift weights 3-4 times a week and try to balance cardio and resistance.  Watch what I eat, weigh five pounds more than I did in high school.  When younger, I was a somewhat compulsive distance runner and had to understand that a day off was often more productive of good health and conditioning than another 10 mile run.