Don't go into it thinking you'll feed all 35K, there are a lot of factors here. Like other food vendors, hours of operation of the fair, weather (the main one) and how much space you actually have to bring in equipment/trailer etc. . and serve from and how much are they (the fair) charging you?
Kev, is right see if you can talk to other vendors and see what the traffic is like, don't go by what the fair organizers say.