In order to better answer yer question, we need to know what sort of AR you are planning on doing.
If it’s a “sprint” (usually last from 3-6 hrs), then if you are reasonably fit from Tri or other endurance training, you can probably just show up and hope for the best. That’s what these races are for, is to give newbies a chance to try them in a relatively easy manner.
If you are doing a longer 12hr or 24hr race, then you very much need to have NAV/orienteering skills wired, or don’t even bother showing up.
Doesn’t matter how fit you are - if you are running or biking or trekking fast, **in the wrong directon. **
If your teammate(s) is/are good at this, then you don’t necessarily need to be yourself, but it is very helpful for everybody to have at least some basic knowledge of this crucial skill. Buy a copy of How to be Expert with Map and Compass, and read and practice it.
Find out what sorta events are part of the race you are doing, and practice them. Trail running and mtb’ing are both givens. Run with a pack on (as has been mentioned before). If you are doing it right, you shouldn’t have more than maybe 15 lbs of pack weight, even for a 24hr race, so practice with less and build up to that. For a sprint race all you need is a Camelbak MULE or similar. For longer races you can’t go wrong with a Salomon AR pack, they totally rule.
Make sure that ALL or your teammates have ALL of the skills necessary to complete ALL of the events. 2 years ago, my team (sans me, I was out that year due to new Daddyhood) finished the entire Balance Bar NY 24 hr AR, and all they had to do was rappel off of the Intrepid into the Hudson river, then climb back up, and they were done. This was after 19 hours of racing, and they were the 1st place all-male team at that time. HOWEVER, my one teammate froze up; he had told us he knew how to rappel, but apparently he was exaggerating more than a tad. He didn’t do the rap, and they got DQ’d and finished with an unofficial result.
Do not let this happen to you.
Oh, and if you have a teammate who is slower on the bike, or run, then it’s a good idea to develop and use a towing system.