Just do what I do, drink and smoke and eat whatever you want to, pay for it later when you are getting shot at and too tired to fire back or notice. I'm joking, really though my first game(this weekend actually
), my legs are tired as fuck today still, you are literally running from cover to cover firing and taking cover agian and running some more. Its pretty demanding on the knees and if you are not in ok shape you'll feel it quickly. I'm in pretty good shape and I was tired by the end of the day forsure.