I know plenty of cyclists who have never ridden more than about 80 miles, and have been cycling for years and years. Probably an equal number who have ridden 100 or more miles exactly once. A buddy of mine has been riding for 15+ years and rode his first imperial century last year.

So as has been said, just finishing it is an accomplishment in itself. And as I've said many times before, people really overthink it. A century isn't that hard. The hardest part for me is mental, because I've ridden thirteen out of my fourteen 100+ mile rides solo and unsupported. If you're with other people, it gets so, so much easier. The miles past 80 when tired and hungry, just me alone with my thoughts... those are rough miles.