Well, when I finally sit on the bench of my piano, I start with exercise and scales but after like 15/20 minutes I get up and start wandering around the apartment and I often start eating..
I would try not to start with exercises and scales but begin with the yummy things instead. 15-20 minutes - well, often I get interrupted in such a time frame, so I consider myself very lucky when I can play for an hour without getting disturbed.
I think you should respect the signal from your brain saying that what you are doing is not the best for you - playing scales, that is. If you do something that is of real benefit for you, you will get absorbed by it.
So, someone, or maybe a lot of people, have told you that it is good to start with scales. That a decent practice session is 45 minutes. That you need self discpline to get somewhere. That you need proper warm-up, or else you will break your fingers when you get loose on Rach 3 or whatever your current project is ...
This is baloney all the way. Baloney because this excellent advice apparently does not work for YOU. You are you. Your brain sends you to the kitchen because it needs a positive fix instantly, and good food is the simpliest and quickest solution ...
Or you may even develop a dangerous habit which you will enforce over time: you start with the usual C Major scale, or whatever, and your brain quickly makes the association: C Major scale -> cookie needed! And then it sends a signal to you stomach and you palate that tell you it's cookie time.
Some of us cannot go to the cinema without feeling the urge to eat popcorn. Some of us react as Pavlov's dog at the sight of a McDonald's sign. A lot of people cannot sit idle in these days for even one minute without starting to fiddle with their cell phones. (And they get a mental breakdown when they lose the WiFi- anyone feeling targeted now?

) It is a planted habit. You need to break it.
So just skip those scales, go for the fun parts instead. If you still get hungry after 20 minutes, then at least you have done something useful ... or maybe you have to accept the fact that you can work for just 20 minutes at a time, good, then do so.