What do you do Frank? Other than racing mountain bikes I mean.
You sound like you are probably in shape for an 18 hour race. That is a good length to start with. The dynamic that makes typical adventure racing so much fun is the team aspect... You have to race with others, usually coed, and all sorts of things come out of that. Talk about team building skills, when you are fatigued and your muscles are cramping and you have massive blisters covering your feet, keeping your spirits up while lost in the dark and helping a teammate who is bonking while you want to move faster is a great test.
Joe (ODB) and I have done some interesting training together in the past. I usually bike a couple days a week, do a run or two, run the stairs in my building when I am crunched on time (I have 41 stories), and then brick training on the weekend, which involves some kind of combined training. I would definitely advise paddling as much as you can until the race. Being a mediocre paddler won't ruin you, but getting into a canoe for the first time with other new paddlers and you will really bomb. I'm not particularly fast, but I can keep the boat straight and hold a pace. I paddled against Joe in a race last year and he beat me by 21 seconds!
One of the best things I ever did to improve my racing confidence is to do an overnight training with a couple of friends. We deliberately started training after a long, hard Friday at 8pm, paddled 10 miles and hiked 24 miles, finishing up around 7am. It was pretty gruelling in the wee-hours. It was harder than most beginner races, so we all knew we could take whatever they would throw at us. That is where most teams fall apart due to stress from mental fatigue, chiggers, and blisters. We all kept eachother up and stayed pretty positive, despite getting rained on and running out of water and gatorade (we did get to restock midway).
Sounds like you have the mountain biking down so I won't presume to advise you on training for that.
Welcome to the forums! Keep us up to date on your training and your race results.