In reply to rob sykes:
Generally I prefer to fuel my long days out with fats and complex carbs with a lot of fibre towards the start of the day, so very small wholemeal or seed rolls stuffed with cream cheese and cured ham, or cheddar and bacon (etc), then moving on to other, simpler carbs as the day wears on, but ideally still "proper food", usually home-made cereal bars or fruit leather, and saving the pure sugar rocket fuel for the last few miles. I like to take a pouch of pure honey for this. The simple sugars really give you a good kick up the arse, but you need to save them for closer to the end or you'll crash.
I also like to stuff a screwtop pouch with melted chocolate, milk, and instant coffee, for a caffeine-packed gel-style fuel.