How to prepare myself for a Gaming Marathon?

Make sure you are well-fed, get some soda/water, or whatever you want to drink, make sure you've already gotten some sleep, and most of all, enjoy!

* Get some tasty but nutritious drinks, ease up on the caffeine.
* Healthy food

Get your blood running, take a break every once in a while, move your neck, stretch, stare at something green for a few minutes.

Dude, some chips and guacamole would be nice. Maybe just chips if you feel it. Coffee? Yes, coffee if that's your thing, should help you stay awake. Muesli bars or bars of some kind; I would recommend chocolate bars but you said not too fat. Biscuits. Maybe energy drinks if you like those, don't drink too many especially at one time otherwise your brain might explode. All about the food ain't I, heh.

Jessica's advice is good; getting some good amount of sleep beforehand.

I would also suggest, if you can spare a few minutes away from the computer every hour or two, to walk about a bit or to madly do some star jumps because that is a totally sane thing to do at 1am in the morning. Nah, just kidding, just to get some blood running and to wake you up a bit. Talking may help you stay awake a bit if you're going to be talking with friends/other players in this game or if you're best friend will be with you in person.

All about the food ain't I, heh. What games are you playing?

A nice meal prior, buy some essential oil peppermint, it will smell good and boost your alertness. A good shower, as for snacks nothing greasy or chocolaty. Try something like chex or similar things. Also, always always, always keep 1-4 glasses of water around.