Yesterday I did my first century. It was on the eastern shore of Maryland, during the second day of Cycle Across Maryland. The terrain is very flat, the temperature was cool in the am rising to mid-80s, and the humidity was low. Perfect.
Except it got up near 90, there wasn't much shade, and it was windy. Really windy. I rode faster than I ever have, going many miles at 14-16 and sometimes faster, but in the end I averaged about 13 mph. The first 60 miles were pretty good, but as the day heated up we took more frequent breaks. I had figured it would take one hour for every 10 miles, including breaks, and that's exactly how it ended up--10.5 hours for 105 miles.
Cycle Across Maryland is an event that definitely skews older, and I'd guess that at least half of the 500 or so participants would qualify for this forum. It's a 3-day ride; I did 30 the first day, then the 105-mile century, and 38 today.
The century was made much more enjoyable since I rode with a 50+ woman who rode across the US (Southern tier) this spring, and we talked about her experience. I have no desire to ever do another century unless my speed improves considerably, as 10.5 hours is a very long time to spend on the bike.
Edit: Here's the proof I did it. I rode 0.4 miles to breakfast, so that makes it 105 total!