Hey Greg,
So I used to do these types of things before I got older and lazy, most of what information I learned from doing it, and from others is purely anecdotal - so take it with a grain of salt.
Like Berkeman is saying, you want to avoid dehydration and hyponatremia, which are both pretty common in marathon runners. You can lose a significant percentage of your body weight in water and salts over the course of the marathon, and you need to replenish both. Sports drinks are probably the most common way to accomplish this, and you should be able to find them easily at any aid station. Speaking of aid stations, if possible, make sure you know where along the route they are before the run, and plan which ones you're going to stop at. If you can time it, try to make your first stop after about 40 or 50 minutes of running, but you might need to adjust this depending on the weather and your sweat rate. When you hit the station, grab two cups. I normally grabbed a cup of water and a sports beverage. On the next stop, if you see a station with bananas or other food items, I would grab one as well. I also use to bring along packets of honey, it's a good treat when boredom sits in. Boredom itself is pretty normal, especially after the first hour or so.
Wear sunscreen, the only thing worse than jello legs the next day is a bad sunburn.
I was always told to avoid OTC pain pills before, during, and after the run. Supposedly they can cause or exacerbate hyponatremia. I don't know if this is true or not, maybe Berkeman would know. Could just be an old wives tale.
You might also want to consider supplementing with creatine, although it's kind of a double edged sword for this kind of running. It increases water retention and you'll probably gain a few pounds that you'll have to drag around with you. It does, however, go a long way in preventing jello legs and promotes recovery. The only times I didn't feel like complete crap the next day was when I was taking it.
Don't drink the sugary sports drink too quickly, it would make me vomit, especially if it's hitting an otherwise dehydrated empty stomach.
Some people take these things way too seriously, so try to make sure you're out of the way at aid stations. Especially if the station already has a large crowd semi-loitering around it. You want to grab what you need and keep going.
Try not to stop moving, it sucks getting into a good rhythm again.
Most importantly, have fun, it's something a lot of people never have the joy of experiencing.
