I have had 12+ hour layovers. I knew ahead of time, so a little different, but still what got me through was exercise. I walked the entire airport route. It was a big airport and it took me about 2 hours of brisk walking. Freshen up in the bathroom as best you can afterwards. Rest, watch movie okay a game. Then walk some more. Keep moving. Time will not fly, but it will feel a lot less overwhelming. And you will have done some good to your body and mind.
Also, work. If you can, work ahead and do stuff you will need to do in the next couple of days. You will enjoy that time you are saving later when you are on your own and need it and can use it.