Race day nutrition begins with breakfast, which you should have the moment you wake up. At this point in the morning you are burning the precious glycogen that you have built up by carb-loading, so you need to get some food in you first thing to slow down that process.
Usually on race day you will be waking up well before the race starts because you'll have to make it to check-in and the pre-race meeting. Ideally you'll have at least 2 hours between breakfast and the start of the race. If you have less time than that, then go very easy on the pre-race meal. Eating carbs can cause a period of hypoglycemia for about an hour or so, so you want to make sure that you are not starting your race off in this state as it will cause diminished performance.
You'll want to eat a pretty filling breakfast before the race so you are not hungry before the start. Eat a good mix of carbs, protein, and fat, favoring carbs as you'll need to replace the glycogen that your body burned while you were asleep (or lying in bed anxiously tossing and turning in my case). But you really don't want to eat an all carb meal. Protein and fat will take longer to digest and help ensure that you're not hungry right after the race starts. And don't forget to drink plenty of water with that food! The last thing you want is to start the race dehydrated.
A key difference between pre-race nutrition for a shorter run or triathlon and for an adventure race is that it's better to err on the side of eating and drinking too much before an adventure race. There are two important reasons for this. First of all you will be working at a lower intensity in an adventure race than a run or triathlon. The higher your heart rate, the higher the probability that digesting food will cause you to have cramps or other gastrointestinal distress. So it's usually better to err on the side of caution for a running race where you will be going all out. But in an adventure race hopefully you're not going all out first thing or you are asking for trouble later in the race.
Secondly, in an adventure race you will have to carry all of your food and water for the entire event (depending on the race style - you can always snag a quick bite and chug a drink at a transition area if the race is not expedition style). So if you are hungry or thirsty at or shortly after the start of the race, then that's just that much more food or water that you will be carrying around in your pack. In addition to food and especially water being heavy, it's also annoying to have to get out and unwrap and eat while trying to make forward progress. So all in all it's better to eat and drink as much as possible beforehand.
Disclaimer: Race day nutrition is highly personal! What makes me feel great during the race may make you vomit. The key is to experiment, tweak, and experiment some more while training. Never eat or drink anything on race day that you haven't had many times before while training. And if something doesn't work for you in one race, then note that and try something different next time.
So you've eaten a good breakfast. Now you need to figure out what food to bring along and how much of it.