Forget the "rules" and go be you. Last year at age 67 I crashed and broke my neck. Had to have C1&C2 fused. this year I am faster and better. Did a Metric Century with 4,000 ft. of climbing at 15.5 mph and then did 27 mi. at 17.4 mph. I'm not saying that faster needs to be your goal but I am saying it's possible, even with age. What's really important is getting exercise and having fun.