I'm 62 and ride 4-5 days a week. 100 - 200 miles a week all year long. Last year only 1200 miles but I had heart surgery and that put me down for 3 months. Now it seems that I have gotten back into fair shape again and am doing lots more than I have in the last 8 years. So listen to your body, push when you can, and rest when you feel like it. Remember to take a nap once in a while. If you are not smiling when you ride, then you are pushing too hard.