Done loads me!
As above really, travel light & be prepared to be uncomortable, travel heavy & be prepared to be uncomortable! It's worth noting that the guys who win carry all their gear on their backs, I don't think there's many who use racks. As for food, all we used was dried pasta, Batchelors tomato & herb etc etc plus your usual chewy/energy stuff for while your'e riding. Another thing which will help you is this, weeks before the event, pack your sack with whatever you intend to take & do a 7 hour ride. When you get home, leave the bag packed then sit on your bike with the sack on your back next morning. This will indicate how your ar$e is gonna feel on the Sunday morning of the event!
See if you can get hold of some Polycarbonate sheet(not Perspex, it shatters easily) about 25cm square for a map board & fasten it to your bars with cable ties through the holes you'll drill through. Those 'Map Traps' that Polaris sell are crap, when your'e hossin down a rocky descent at daft mph, the map WILL fly away. Fasten your map to the board with a clip from a clip board & relocated to the Poly sheet & a big fat elastic band at the bottom. Get a waterproof map, don't use a paper map in a bag, it'll end in tears (Pun!)
By Saturday night you'll have learned a lot!
Enjoy & have fun.