Why are you worrying about doing 'centuries' when you have so much pain after riding? It seems like you need to relax and do some stress less 20-25 mile rides and see at what length and speed you can go where you do not hurt afterward. Then you can slowly build up. I am also more than twice as old as you. When I ride in the summer, I can go farther and more frequently than now - when the rain interferes with riding. I don't get really sore, but my muscles feel fatigued. Than I take a couple of days off. Or swim or water aerobics - it's great for stretching. I know I will be able to do more in the Spring. Just take it easy!