Just keep going. There's a week to go, and then it'll all be over - you can do it!
As someone who recently submitted a PhD, my thoughts on trying to stay sane/healthy while finishing...
- do try get some exercise in, like Klix said, even if it's just 20 minutes. I think swimming & jogging kept me sane in the last few weeks!
- take regular breaks when you're working. If you're finding it hard to focus, try something like the Pomodoro technique, where you work in 25-minute bursts with 5-minute breaks in between. A week out from submission is probably not the ideal time to be working out a new productivity technique, but it could help you get through those patches when the words don't seem to be flowing, or you're having trouble editing a particular passage, etc.
- make sure you sleep & try maintain a regular sleep schedule. I know you said you were a bit all over the place and that's understandable, but do try make sure you get some decent shuteye each night - don't be tempted to pull all-nighters!
I wouldn't worry about gaining weight or the fact that you're eating a bit crap at the moment. There are only a few more days to go, after which you can work out a new eating & exercise regime for yourself, and anything you did gain (if indeed you did put anything on) will quickly come off.
Good luck!