If you've got time, go out on a long road ride. Bring plenty of food. Eat what you think you need and see what the result is. Adjust for the race.
Not having ever raced, I can't comment more than that but I have done a few long (6+ hour) road rides and I learn something new about what I need each time. One of those things is to make sure you are properly fueled before the ride starts. I've found Post shreaded wheat cereal to be fantastic before a hard ride. It's ALL complex carbs.