Good advice above. I'd add that there are some (I'm one of them) who think that there's minimal benefit to training beyond about 200K. Physically, if you can ride a 200K brevet within the time limit, you're not likely to have fitness issues on the longer brevets, including -- I'll make the bold statement -- a 1200K. The issues that you've got to deal with are mental and logistical, and both can be solved (although moreso the latter) by getting out there and riding and by talking with and reading the ramblings of more experienced randonneurs.

It may sound counterintuitive, but I think one way to train for the longer brevets is actually to train yourself to ride faster, which means doing shorter rides. You've got the distance bug. You've got good base miles. Saddle time? Check. Mentally, you're both tough and flexible. Logistics? You've got trial-and-error combined with lots of internet info gathering plus chatting up every randonneur you've ridden with about what works for him or her. An equally important arrow in your quiver is to get a bit faster. Buys you the luxury of not having to worry about controle closing times -- huge mental benefit. Buys you the luxury of sleep on the longer brevets. Also buys you the luxury of less night riding, which can be slower, more dangerous (especailly if you're solo) and cold. Increases the number of people you can ride with, which means more riding companions.

I'm not talking about going out and training to ride Le Tour or anything. Just increasing your average rolling speed by 1mph means your 600K just got 2-3 hours faster, depending on where you're starting from. That's huge!

Good luck!