Man, I don't get enough exercise while I'm writing during NaNoWriMo. I doubt anyone really does.
But it's important. I recommend getting up and walking around every forty-five minutes or so. Yeah, I know you're in the zone and breaking that to take a walk seems counterintuitive.
It makes sense, though.
For me, when I'm working - writing - I have to move around. If the weather's okay, I may take a walk around the block. Just ten or fifteen minutes to lubricate the body so the mind can work faster. If it's crap outside I'll walk from room to room, or at least stand up and talk to whoever's in the house for a bit and go wash my face or something.
What I want to avoid is just being a sitter, static, while I'm writing. If I'm moving around there's more likely to be movement in the story. Weird but true.
Ritualize it if you must, go for a jog or head off to the gym, but exercise every day while you're writing in addition to getting up every forty-five. Get your blood pumping. That really helps you think.
Don't take my word for it though. Ask your doctor when you get sick and have to go see him next time. He'll tell you, too.
Anyway, busy writing. Updates to follow.