Go have fun. If you stress too much about it (or let others stress you too much) it will get 'old' real quick.
If the location/access permits try and take all and any gear you think you might need (even if most of it stays in the car a couple hundred metres walk away). Your mates might think you're mad until you roll out that perfect solution to the unforeseen problem (eg a spare chair for the mate who forgets etc).
Going through and laying out the gear before packing, while checking that you have all the basics helps. It can be good to jog you're memory about something you might have forgot. As said before, being ready for any weather (dust, rain, wind, sun etc). A 4man 12hr at least means you can change after one lap in pouring rain!
When 'training' before the event, start testing foods, sports drinks, gels etc; anything you might think you'll eat on the day. Avoid trying something new on the day of the event. It's not fun if you eat something that doesn't work for work for you on the day - even one gel compared to another can be an issue for some.