When training for a double, you have to not think about distance when you are training. Doing a century shouldn't be a big ordeal, and should be a common weekend ride. Throw in a few double metrics, and some 150 milers, so that you build up your stamina.
I did 3, and only 3 last year. It does take a lot out of you, and I just didn't have the motivation to go for a 4th and 5th one. I just concentrate turning my crank!
Oh, and I agree with Princess Zippy ... Solvang in Spring is easier. However, Davis support is the best.