My dad used to do triathalons a lot and the whole family would carbo-load (pasta mostly with us) with him for the week before each race. We couldn't stand to eat spaghetti for a few months after each pme. Though a triathalon is more of an entire day's committment, maybe some carbo-loading for the last couple days before your test might give you the energy to help you get through that intense half hour. Best of luck!
P.S. I know carbs aren't the "in" health food now, but you sound like a healthy guy who's not necessarily trying to lose weight, and just a couple days of pasta won't ruin your metabolism and/or body-image