Ok, I`m a doctor, here is my advice.
1) Playing that long could be dangerous. I`ve heard stories one playing much without stop, went out to the street, where he collapsed and died. I can`t suggest doing it from a medical standpoint.
2) It seems you can do that much, but in reality it is harder than it seems. I`m a competitive FPS player, once I played many matches in 5 or 6 hours marathon (forgot) and was tired as hell. I always played with really good players, and I won a Tournament too, yet was tired. My dueling opponent too, and he was a hardcore gamer. I am privileged to know personally one of the greatest players of Quake in the world, never saw them trying to play 30 hours in a row.
3) Sleep is a key factor. If you really want to do this, sleep before that as much as you can. Even more than 10 hours. As much as you can.
4) I don`t know if you allow yourself breaks. Breaks would help you to relief.
5) It is important to have plenty of water. I don`t suggest fizzy drinks. You can take multivitamin effervescent too.
6) Food should be mixed and have a greater variety. Might sound funny, but including vegetables and fruit is also a good idea. They will also keep you fresh and feel better, which might have a very little impact on your gaming skills too.
7) Let in some fresh air and natural light if possible. Open the window.
8) If you feel really bad break off immediately. In case of feeling really bad don`t attempt to stand up of lay down too suddenly. Do everything slowly without haste.
9) I bet a current record holder went through some preparation before he started this. That means getting used to playing more gradually. It`d be harder without preparatory training.
10) I wish you all the best. Sorry if my advice sound strict, but I want to advice only what I see is the best for you.