believe it or not, I had been riding for 8 years before I did my first one. I was on a 75 miles with some friends, we made a wrong turn, and decided to make a 100 miler out of it. So I'm not much help there
It sounds you've just been done shorter rides on your own. I'd try and work in some longer rides over appropriate terrain.
Also, Registration for the Amtrak Century is this Friday. I'd think that would be good local one. Tour De Palm Springs is a another. I've also heard good things about Cool Breeze.