just rest from now until sunday as there is no time for improvement but plenty of time to rest. Plan to do the first 5km if you need a plan, but if it doesn't turn out that way oh well. Time it but dont have a set time you want to do it in.
carb up, not crap but good carbs pasta, rice etc, and eat well the night before and of course lots of vegies.
at worst it will be a training run. and don't worry about the other runners, if it worries you just put your ipod on, it may annoy some people but it's a long way you will need some music to help you along.