Despite being off the bike for two months in April and May because of emergency surgery, I managed to get in 19 100+ mile rides this year. I think about 5 of those were organized events. The rest were all solo rides. My last century was last weekend. It gets harder to do the longer distances when the days are shorter though.
I don't enjoy all of these rides though. Some are really not fun at all. Thankfully most of them are.
In some cases it comes down to riding a century in order to be in good enough shape to ride a century....