Mixed carbohydrates: combo of simple sugars and maltodextrins. I find at that somewhere between 35 and 50 miles I will get early signs of the bonk. It can be difficults to distinguish between bonk and dehydration, both will make you feel bad.
If you plan a longer ride, eat a good solid meal a couple of hours before and a snack such as Powerbar at the start. Preemptive Powerbar (or snickers for me) at 30-40miles will power me for 10-15miles, then a 100-150 cal carbo snack every 10miles does it for me. I like Fruitopia or similar at 40-60mi, gives 20oz fluid and lots of simple sugars for fast energy- at least 150-200cal. Plan for 1-2oz fluid intake per mile: about half gator/power/acceler*ade, half water with the two oz level for temps above 85-89F.
Steve