Apparently this is how you do a century: you go out and ride 100 km. I really don't know why I didn't figure this out sooner.

EDIT August 17: I originally had this as 5h5m45sec and an average speed of 19.73 km/hr, but this was based on turning on and off the stopwatch, as I don't have a ride time display on my computer. The stopwatch doesn't throw a lot of error into a small ride but does screw up longer rides.

So I've recalculated the duration, which shows that my pace was significantly slower, based on the actual km/hr and distance registered on my computer. (Yes, I took photos. Hey, this was my first century.)