For a 24hr race that I did, someone brought a mini-camper ('Scamp') and it seemed like an awesome way to keep warm without having to own a RV.
I would probably try to eat a big meal before I got there and then tried to eat filling things throughout the day. I know with me, if I eat a big meal and then stand around outside, my extremities will get colder faster.
Soup, herbal teas were all good suggestions.