The posts here show a pretty wide range of opinions. All people are different in how many cals they need per day, but I agree with every single post about water. You really can't drink too much when it's warm outside.

Dehydration will kill you. I don't understand the juices, gels and bars. As already stated they are expensive and have too much sugar and for the most part are pointless.

Always have more water than you need and always have some easily consumable food with you. My favorite is Mars bars, but they don't travel well in the heat. My best advice is always have something with you that you like to eat. If you don't like it, you won't eat it and are more likely to bonk.