I've hit the 5 hour mark, for riding time, twice. I don't even look at mileage, but as the only time I roll at <20mph is for a recovery ride or with a crazy headwind, I'll assume it was 100 or more. Not to mention it was with a group of 5 other guys each time, which would further increase the pace.

I normally ride for 3 hours or more once or twice a week, as well as whatever the racing schedule throws at me, i.e. road races.

And, honestly, distance really doesn't mean much. A point to point RR with a tailwind, even at 160km, is pretty easy if you aren't doing any work. Whereas a 100km RR with several rated climbs on it is going to suck, regardless of whatever meteorological assistance you might recieve.

So, riding centuries doesn't really appeal to me. I don't see the point.