At 66, I can ride a flat century (no hills in my area) or more with no pain at all. Of course some minor discomfort, like feeling I'm sitting on a saddle after 3-4 hours, and maybe my legs feel a little stiff while descending stairs that night, but nothing more than that. My longest distance ride was 365 kms, longest time 16 hours. Felt tired the day after, but no pain.