You could go with skinnier, higher pressure tires that might gain you 1+ mph. Perhaps 700 x 28 ??
My mtn bike w/ 26" 1.75 tires is about 3 mph slower over the same route as my road bike with 700 x 23 tires at 100 psi.
Also, can you start towards the beginning of the event to give you a bit of a head start over the main crowd ?