Jamesy, done this three times now last one in 2004, don't go getting any big heavy boots, get some good light trekking shoes, and get them as soon as you can, then the only really good training you can do is to get out there and cover lots of ground especially hills the steeper the better.
Obviously to start with build up steady, a couple of 5 milers over a week then start making them longer. If you know that you get blisters, before you start any walking put some "Mole skin" on the spots where you get them, use only the best socks, (I use bridgedale trekker) and I would have at least one fresh pair for each lumpy bit.
The three main bits of advice would be, get out there and cover a lot of ground, you havn't got long till July, look after your feet, and make sure you have a good support team and drivers. Oh and enjoy it's worth doing.