I will be taking 4 gels with me I think, as in training I have used 1 every hour. Also maybe some chocolate covered coffee beans which are nice. I have decided to take my camelbak as I don't want to drink other sports drinks etc. My boss at work used energy drinks provided at the London Marathon which she hadn't used in training and it gave her stomach cramps from mile 18 onwards. Stick to what you know seems to be the best policy.