Use your 20 mile run next week as a practice run for the big day - wear the same clothes you plan to wear on race day, eat the same pre-race meal, use the same nutrition products (gels, sports drinks, etc) you will use on race day.
On race day, as others have said try to start out slow - make sure you drink enough and then take it in and enjoy the experience. I'm quite jealous. I ran my first marathon here in San Diego this year and would love to run New York one day.