I used to ride a solo century every year since about 1987 (never, ever sub-5 hrs or even 6). It seemed more a cycling obligation than a joy. Upon turning 62, I decided all my future centuries, would be metric. About 2 hours at a time in the saddle is all I find enjoyable, then break, then ride again.

Most of them were: Santa Cruz to Monterey, CA and return = 100.01 miles, about as flat as one can ride in coastal California.