funny you should post this b/c I am wasted today and wondering why... I'm almost 55 y/o and am riding about 100 miles a week, and sometimes - I don't know why - just dropping after a ride. I do a lot of major hills (live on a super-hilly island) so the mileage isn't all of it - I can do a brutal 20-mile ride with 6 substantial hills... At any rate, I did five consecutive days of 20-20-20-20-30, and today I'm empty. How much does age factor in?