+1 all good advice.
My 2 pence would be don't stuff yourself at lunch, just keep topping your blood sugar up after each session so take loads of tit bits, as others have said remember to drink water after each session as you will sweat more than you realise. Also just enjoy it, take it easy first couple of sessions get to know the track, you have all day, it isn't a race it is a track day so don't worry if people are passing you it doesn't matter. If there are instructors there get a free session with one of them, ask them to take you out and check your lines etc. Finally short shift before you hit Gerrards.
Enjoy it as it is a fabulous experience.