Stack on on any carbs three days before, i.e rice, pasta, spuds etc.
On the morning eat a light breakfast and drink a pint of fluid.
Half an hour before the run, eat a banana and a small can of ANYTHING sugary. The ones served on airplanes size.
Although 5 miles is not really enough to dehydrate, if you feel lightheaded or sick during the race stop immediately and drink water. Drinking some throughout the run should prevent this, but not necessary UNLESS an absolute scorching morning.
However after the run replace your fluid levels with lots of water.