Well, I think the main thing you need to do is get running. Firstly, you'll be able to work off some of those fat-boy pies (they do sound nice though!), and you'll build up your fitness, which is one of the main things that will carry you through a competition, especially if you have several rounds to do.

One of the quickest methods to get fit I found was to do the jog-sprint-jog (use lamp posts as your marker)for about 2 miles. It's knackering but effective. Find out how long your rounds are for and practise running for that length of time as well, as that will give you an indication of how long you have to last.

Keep sparring as well, up until about a week before the competition. Afterall, that is what we do!

Good Luck, let us know how you get on.