I have completed nine 300K brevets, and a whole bunch of longer brevets and randonnees.
If you'd like, you can read about some of the 300Ks, here:
http://www.machka.net/ - scroll down to the "Brevet Stories" link.
The biggest piece of advice I can give you right now is: EAT REGULARLY WHILE RIDING. You mess that up, and it'll be game over for your 300K ... or if luck is with you, you'll finish, but you'll feel really, really horrible.
Since you've got a month, I'd also suggest riding regularly. Go out 5-6 days a week. They don't have to all be really long rides - a regular 20 miles will go a long way to building your fitness and making you feel more comfortable. Also, once a week, ride a long(ish) one - if you're up for it, go for a century this weekend, and aim for at least a couple metric centuries at some point before the 300K.
Also remember, although these are timed events, they are not races. You do not have to ride them at top speed. Ride steady instead, you'll last a lot longer that way.
