I must have not seen it when you posted in the AR section... sorry!

I got into it by hearing about one and jumping and and doing it with a friend. There are a number of sites out there that post info, as well as a number of local groups, which often have yahoo groups. I only know the California groups and the general adventure racing one though. Looks like http://www.trailblazerar.com/ might be in your area? My geography sucks... I just found it through a google search for AR in the Raleigh, NC area. I seem to remember hearing good things about the races put on by http://www.oarevents.com/home.asp, but am not sure how much you want to travel or how far they are from you (hey I got the coast correct...).

There is also an event calendar at AdventureRaceReports.com (not to mention some great writeups as a way to learn a bit more about races or organizations you are considering trying).

I'd say the biggest thing for an AR is finding a team of people to race with (if you are doing a team race). The team dynamic will make or break the race -- for performance, satisfaction, and overall experience.

You'll need a mountain bike and just a sense of adventure... oh and a pair of shoes might be good, too.

AR is tons of fun! Check out checkpointzero.com and adventuresportsmagazine.com for some calendars. Just grab some friends and give it a try. Some race directors/race websites will help match teammates if you can't find anyone. When you get into it more you'll meet enough people and you can worry more about forming the right team- matching personalities, goals, abilities....
Don't worry too much about training- if you're doing IM you'll have no problem!