You don't need that much in the tank to race CX. It's too short and intense to have a big heavy meal sitting in there. Make sure you are hydrated starting 3 days before the event.
I usually eat a couple mini wheat bagels with Nutella around 2-3 hours before go time. Drink some water. Maybe a gel like others have suggested.
I'd warm up for at least 30 minutes, with some hard efforts in there too. If you are worried about getting worn out during the warmup, then you aren't in good enough shape to race. And the shorter the race, the longer the warmup.