If you're comfortable with doing a 200km and you've already finished a 300, then you're all set for physical conditioning. Everything after this is learning to eat and drink properly and dealing with sleep deprivation.

Figure out a personal schedule for taking in enough calories every hour so that you can ride 200km and still feel fresh. Do an overnight ride where you start at 1am or 4am. Also, do a pair of longish rides back to back on a Saturday and Sunday to get yourself used to getting back on the bike after a short night's sleep. If you want to do a 1200k eventually, a lot of folks advise doing a 600k brevet without sleep so as to simulate the level of sleep deprivation that you'll be getting on the 1200k.

After getting my 300K under my belt, my training was pretty much consisting of 200k's on available weekends and one back-to-back pair with a 200k ride on Saturday and a 100km ride on Sunday. And, oh yeah, regular commutes to work (roughly 50k round-trip).