I haven't done a GNCC but I have done a couple Mid East which are basically the same. My suggestion is to start in the lowest class you can and give it a shot. If some of the guys I know can do it anybody can. Just remember to ride your own pace and the main thing is to finish. Get a few under your belt then work on getting faster. Remember it's an endurance event not a sprint only go as fast as you can maintain for the entire race with a little in reserve for the end. Good luck and have fun.
ETA: I usually attend Steel Creek and Big Buck so if you decide to run them and need some help in the pits during those two events give me a shout.