Our test was very long too, but I'm not Kenpo.
Nutrition-wise: Don't mix your starches with animal proteins, take vitamins to include some E (check with your doctor on the dosage for you and if it's indicated) TW is right, carb-load the night before and bring an energy bar, a protein bar and fresh fruit & crackers to your test with a good elctrolyte replacement and water. I brought: Orange, low-fat wheat thins, raisins, banana, (forgot the protein bar, so I borrowed one), PowerAde and water.
Exercise-wise: Work out EVERY DAY until you're almost ready to drop, but be careful and don't injure yourself when fatigued. Wait a couple of minutes, then do a little more. Train for everything your expected that you're aware of plus 50%. IT'S CRUCIAL TO TRAIN YOUR BREATHING AND FOCUS. You're going to get out of breath on your test - control your breathing when you train and in your test. Train your legs - A LOT!
Mental training: Visualize yourself in the first person performing everything perfectly, calmly, clearly and with ease over and over and over again, without fatigue. When you're done with your meditation, feel great about how you did in your imaginary test and feel confident that you will perform just as well on the real one.
Just so you know - your REAL black belt test is in your preparation ... how you get there. The test has already started. The gauntlet you will go through that day is a rite of passage - a formality.
You'll do fine. :asian: