1) Vaseline your toes, between your thighs (groin area), under your armpits.
2) Carry a small amount of Vaseline in your running shorts pocket or running belt, either a small toothpaste-like container or wrapped in saranwrap.
3) Drink plenty of water at the stations, and if hot, soak your head to cool off.
4) Bring a small package or packages of tissue in your running belt or shorts' pocket for the toiletries, which may be out of paper when you arrive to use the facility.
5) Wear shoes that you have run in a few times, or have walked miles in, but are not too worn. It helps to have the bounce of newer shoes.
Slow people can be expected to run 10 minutes a mile for the first 10 miles.
After that, you will be jogging and walking, because around 30km (18 miles is around 29 km) is when most people are getting in over their heads.
If you make 20 miles reasonably, expect to walk much of the last 6+ miles. If you c an jog the last part, great for you!
I've made the mistake of running 2 marathons.
First I ran without any water, and sped up the last 7 miles, both of which were mistakes. I found out later Pheidippides probably didn't run the 25 miles (1908 London Olympics extends to 26 miles 385 yards), but he likely ran from Athens to Sparta in about 40 hours (amazing!).
As an aside, I ran 8 min miles for 20, then 7 min miles and finished a few minutes over 3 hours. The race was in the city where I went to high school, and I was spotted in the area after the finish line by a runner who remembered me from high school (he went to a neighboring high school) and wanted to know what interval training that I was doing for shorter distances (mile, 5k, etc). We talked and then he gave me a ride in his car to the start area, while we continued to discuss interval training. The radio was on, and announced that he was the winner of the marathon! I didn't know, partly because I was so far behind.
Second, in 1988 I ran in the Malaysia Marathon, serving as their national championships. l remember that Doug Kurtis won, and that I got my photo in the local paper because at the 5 mile mark I was with the Malaysian Marathon champion (and a Japanese runner). We were 25:25 at the 5 mile mark. I crossed 20 miles under 2 hours, but walked most of the last 5 1/2 miles for a 2:57 (unofficially 38th). I promised myself not to run another.
best wishes