When you are 6 or 9 months away from your trip, it's time to buy your gear and start using it. You want to build up to at least being able to spend 5 or 6 hours in the saddle. You want all your tack really broken in so that you and your gear are in condition to ride those long hours.
Get familiar with a good rope - another piece of gear that is essential to ranch work. You want to be able to handle it off your horse and you'll need to learn how to dally.
It's pretty exciting and it'll be a great experience - one that many riders never get to have!!