I'm in the same boat; I love backpacking, trailrunning, bicycling, and kayaking, have done all of them for years, but not at what I'd call competitive levels of athleticism. I've not entered the fray because of being unsure of the base level of fitness required to be a meaningful team member.
I'm definitely looking for books as well - the Ultimate Guide to Adventure Racing gets good marks at Amazon, as do the Complete Guide and a couple of others.
Being an utter novice, too, I am considering joining a local group (here in the SF Bay Area it's BAARBD - Bay Area Adventure Racing Babes n Dudes) to learn more...heck, for kayak rentals and gear buying the membership will pay for itself! :-)
Anyone with experience care to chime in to guide us along?