I'm pretty sure you can just show up and ride (since it's free) - but it would be a good idea to pre-register online to avoid lines/etc on the day of.
Also you might be able to get your RUSA # from the rusa.org website now (if you want the pop. to count towards your yearly RUSA stats); they might have your RUSA # in there but not mailed you back your stuff yet, if you've already mailed it in.
Sounds like a fun/hilly/tough route! They're a great group to ride with, and always pick great routes... though at somewhere around the 400k mark on a 600k last year I inevitably cursed the organizer, e.g. "who would create such a stupid route??!" - but it was usually just a sign it was time to eat again.
Btw lots of posts of my rando adventures from the last two years here on my blog
. I'll probably show up for a few brevets this year too.