Most people who are serious cyclists of any type will probably do a century within their first few years if they keep up the hobby. I got back into cycling in 2008 and did my first century in '09. Haven't done another century since then, but I've done two club rides that were metric centuries, the most recent of which was this past weekend.

While long distance riding obviously involves athleticism, for those who ride with any regularity, the trick isn't so much about the added mileage, but proper eating and pacing. People who do long distances regularly get a routine down so that they stay fueled and hydrated properly and pace themselves so that they don't send their muscles into overdrive before they're warmed up. The latter is one of the reasons you'll see many Gran Fondos will have a neutral zone for the first 10-20 miles before racing really begins.