This close to the race, just get plenty of sleep and rest over the next few days. It took me a few marathons to realize this, but what you put in your body the days leading up to race is also very important. There's plenty of carbo loading theories out there, but it's almost more important to stay away from anything that can wreck havoc on your digestive system. Having to unexpectedly use the bathroom a couple times can derail your race very quickly.
During the race, be proactive with water and any gels/goo/whatever energy source you'll be putting in your body. If you wait until you cramp up or hit a wall, it's going to be hard to recover, especially late in the race. It sounds a little cliche but taking the race one mile at a time has helped me in the past versus "counting down" how much longer you still have to run. Having a goal time is a must, but as a first time marathoner be prepared to have an A, B and C goal for your finish time as there are a lot of factors at play with how well the race can go.
Good luck! Running a full marathon is a big deal and a heck on an an accomplishment. It's a great feeling finishing that first marathon but be careful, you might get hooked on it like me