It's much easier to tuck in behind someone and let him/her pull you along than it is to maintain the pace on your own, so let other people do the work for as long as possible, especially if it's windy. The oxygen cost of running (and therefore the perception of effort) increases when you run into a headwind. Let someone break the wind for you.
When you run for long periods of time, you can get chafed in places you don't want to get chafed, which can make the marathon miserable. Apply BodyGlide or Vaseline before the race to any place that will be rubbed up against, such as inner thighs, nipples, and below your armpits.
Although the urge to go to the bathroom is often suppressed while running to conserve water, nervousness and anxiety often intensify that urge, so take care of business before the race.
RehearseThe day of the marathon is not the time to do anything different. Do at least some of your long runs in the same clothes and shoes you plan to wear for the marathon. Don't buy new shoes to wear in the race. Rehearse everything—shorts, socks, shoes, what you plan to carry on you, etc. Leave nothing to chance. Practice drinking water from a cup while running.
It can be overwhelming to think of running 26.2 miles all at once. So divide the marathon into smaller segments. Focus on each 5K or even each mile at a time. If you're aiming for a specific time goal, focus on attaining that goal at each 5K checkpoint.race.