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One 20 minutes before the start and another after 40-50 minutes, you shouldn't technically "need" to take on board extra energy but there are plenty of reasons why its not a terrible idea, but if you haven't got your stomach used to trying to digest things while running reasonably hard then you may give yourself a case of runners trots if you over do it.
nobody wants to do a Paula...
http://guynonymous.files.wordpress.com/2008/07/shittyrun.jpg
Surrey. My target time will be somewhere between a (just) sub 1:40 and 1:50.