I've done some centuries - some we create (there's a spring ride 20 miles from town, so we ride out and do their supported ride and ride back; comes out to 90-100 miles, depending on where you live). I'm not a slave to the number 100 - if 95 miles is what I've done and I'm *done,*, well, I'm done. Too many people break bones on that last ski run.
Centuries take a long time - I know many devoted riders who just haven't done them just because it hasn't come around on the guitar, so to speak. I know one guy who did one on a brutally hot day, so now he says he can't do them. Argh.
I've also done a double metric and the Ride Across Indiana - it's a good feeling to be strong enough to do that. It so didn't happen overnight, though it did sneak up on me- it didn't happen because of grueling discipline either, just getting out and having fun riding.

I *love* the Peanut Ride. [long reminiscence coming:]

I've even driven there from here for it twice. I arrived at dawn at the motel where my friends were staying and a cop followed me into the parking lot... but once I got out and looked at Clyde's car, which had some encryption that was supposed to tell me which room they were in, he drove away. I took an hour's nap in the front seat and then we rode my longest ever ride then, the Lake Gaston ride. I was so afraid that sleep deprivation would propel me to bonkville that I paced myself really carefully... so I was fine.
THe next day we did the metric (63 miels) and I'll never forget realizing jus thow much progress I'd made. My first and second metrics were both at the Peanut Ride in subsequent years; the second time we at least got back in time to meet the 3:00 official "Peanut Ride" halfway... but both times I was pretty pooped and it was definitely a ride from one rest stop to the next (never more than 12 miles). I'd averaged 11 and 12.5 mph respectively and proud to finish. (Oh, and I wasn't alone back there, either ;-))
This time we came out of the 50-mile rest stop and we were on a crowded hill ... I didn't like it! So I pulled out and charged to the front... and heard a women say in something almost like awe, "Oh my G*d!" I looked down and I was going 17mph... uphill after 50 miles (on a heavy hybrid, too, but I didn't realize that added to the effect)! It wasn't a big hill, and I didn't have a *whole* lot left... but it was there