Many of the century rides will publish their routes, and you are welcome to do the rides self-supported (or with a small group if you wish).
I'm down on my miles this year, but over the past few years I was a hardcore commuter, and had far more century commutes than organized century (and hilly half century) rides. I'd rather do a point to point trip that I'd like to do for a commute than to simply ride in a circle for the miles.