The biggest thing I've read is that I should take a 10 minute break every hour to keep my heart/veins from imploding.
lolwut, that's a 16% productivity loss.
Seriously, health risks are always ludicrously exaggerated on the internet. Your body will be fine, and it will let you know when you need to stretch or go for a walk or have a drink. It isn't retarded and neither are you (if someone is stupid enough to ignore all warning signs and stay put for days on end glued to a computer screen, then one deserves to die - no offense, it's just natural selection at work). If you are in decent health now you will not die or be permanently damaged from an occasional gaming marathon. It just becomes a problem when you do that too often and don't give yourself time to recover.
On a related note, my last exam is tomorrow. I might have a marathon of my own next week or so