mark, thinking you are getting the wrong idea about sugar? Sugar is a fast and convenient way of getting fuel into the body. Unfortunately its a bit like crack (allegedly?) A quick fast hit followed by a massive drop. Nothing wrong with that (sugar not crack) as long as you are expecting that to happen.
i.e if it is say 1 1/2 hours then sugary energy is possible as you just need to keep topping up. If it is 12 hours then sugar will be bad and then you need a mix of slow release (oats and generally low gi food) and sugary crap (the good stuff)
Also as mentioned, if you do go a bit wonky then a handful of jelly babies and a gel will get you going again.
Oh and thinking some more, Banana and honey bars, rice crispie toffee squares (one of the highest carb to weight snacks you can find in a morrisons - Fact!), bogoya bananas, almonds.
One of the main reasons I tire is hydration or lack of, as I def eat enough.
And my not so secret weapon at mountain marathons or after a heavy exercise session Pork Scratching. It replaces lost salts, and high in calories and protein to aid recovery for the next day. Okay I know the science may be floored and if it is don't tell me why as I like to keep the idea of pig skin being one of the good things in life, sneakily packaged as a pub snack 
