In reply to Jamie McCreath:
Could not agree more strongly with previous post - hill fitness and more specifically climbing steep hills quickly will be your best training.
Running is good for your endurance though - once your body is used to longer runs, you can start making bigger increases in distance but more importantly time - slow and steady over long distances is much better than fast and short.
Depending on what class you enter, you will soon realise you may run relatively little, unless your planning on winning one of the top classes!Having the strength to run when things flatten off or the terrain becomes less gnarly is critical as is being able to run down hill.
Take a pack a few times as well, dont overdo things but at least get used to your pack, making sure it isn't going to destroy your back and the extra abuse this will put through your body.
And practice navigation! Remember spending 5 minutes looking at the map could save your hours of running of time in the end, so dont think you are under massive pressure to make split second decisions to begin with!