In reply to paddyposh:
I would run only in trainers. Running in boots, with kit puts too much stress on your body, especially your knees, but also lower back.
The trick is to build up your cardio. Do some core exercises to build core (obviously) - many injuries carrying kit are due to posture and weak upper body.
I would then add in as others have said, increasingly longer days in the hill, carrying kit.
As the event gets closer, try a few easy runs with a bergen on to get used to it, but just don't beast yourself.
Pcoy, and the training for it, is a war of attrition - the ones who rest as much as train, and take it in sensible structured chunks are the ones who tend not to get back squadded.