you could try to carbo load the day before at about 5g/1lb of body weight spread out over your normal meals.
you should eat a good, hearty meal of a mix of carbs/protein/fat between 3 hours and 1 hour before your test. Figure it for 300-500 grams of carbs. good carbs, not high octane sugars.
if it's possible to drink during the test. go for gatorade or some other sports drink. they will have water/carbs/salt in appropriate amounts. somewhere around 1g/1lb of body weight per hour will keep you in good energy. read the label. as you will want up to 2 quarts of water in an hour (depending on heart rate/sweating) you may not want to take in those 2qt/hr in all gatorade and could cut the gatorade with water.
good luck.
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