Between work, a visiting mother and TE outages I didn't see this thread til now!
Yay! We got another one!
You've gotten loads of great advice, CC. Definitely work on building up your long rides. 200k is a great distance to start with for a brevet, to see if you actually like it and want to ride farther. Most of it is mental. Seriously. You need a lot of practice doing long rides mostly by yourself to see how this works for you and if it drives you even crazier.
In terms of bags, I use the Tailgator as SK mentioned, and I also have a large handle bar bag, but for 200k I don't use the bar bag.
Let us know how the training goes!