The day before a race, first focus on your fueling strategy. If you haven't yet done so, plan your race fueling strategy. How often do you plan to take in fuel and hydrate? Do you have proper gels and blocks, and the equipment to carry them along with you? Do you need to go ahead and prep your breakfast ahead of time so that the early morning race alarm does not leave you with an entire checklist of things to do still, like boil eggs and package up your starting line bagel. Next, get in enough carbs. You should be aiming for 60-75 percent of each meal to be carbohydrate rich. This is how you will have the energy to fuel you through that marathon, and help you conquer those tough hills and stretches of the race route! But do not try out NEW carbs. The biggest mistake new runners make is trying to change up what they eat the day or days before a race - risking possible GI issues and stress that could backfire on race day.
As far as running is concerned, you really want to take it easy. Make sure you do not plan any big events for that day, especially stressful runs, and if you really feel like you need to get out for a run, keep it short, keep it light, and focus more on just "shaking out the jitters" than completing a real training run. You will be giving everything you have the next day, and you want to be sure you have something TO give.
Finally, lay out your clothes and gear so that they are easy to find and so that you can put them on quickly. Again, that early morning race day alarm can come REAL quick. For your mental sanity and so that you feel as least stressed and chaotic as possible on the day of the race, try and get done as any things as possible the night before. And then, before you go to bed, do some mental exercises that help calm your brain, and get you really mentally prepared for the race. Meditation techniques are great too, especially if you are too wound up to fall asleep. Trust in your training! You'll do great!