Log as many miles running or biking as you want, but skip training with a pack and you will risk being unprepared for your climb. People might want to forgo this type of training for a number of reasons. Sometimes you may not have access to a pack, especially if you plan on renting one. Also, people can get embarrassed at the thought of hiking around town with a weighted pack, or feel even sillier strapping on the pack to spend time on the elliptical at the gym.
It's very simple though. No other exercise or training will prepare you quite like hiking up some hills with a loaded pack. You will become comfortable with the level of exertion that hiking up steep terrain with a pack requires, as well as developing the same muscles that you will use extensively on your climb.
The key to training with a pack is to start with light to moderate weight and gradually increase as your training regime progresses. Gradually working up to what your pack will weight during your expedition will insure that you avoid injury. A favorite tip about training with a pack, is to use water bottles to create a lot of the weight of your pack. That way, if it gets a little heavy, or you want to reduce the weight on the descent, its as simple as pouring out some water on the trail.
Training with a pack also lets you assess your pack's fit and performance. Is it the right size? Are there any problems adjusting it? Does it fit all of my equipment inside? Are all questions that can be answered during a training hike. If you are planning to rent your pack, I highly suggest borrowing a pack from a friend to train, or at the very least, loading up the biggest backpack you have to attain some of the advantages of training with a loaded pack.
Short on hilly trails in your area? There are other ways you can train with a pack that don't require a hill. Climbing stairs with a pack will offer some of the same benefits of strengthening quads and calves. I would recommend reducing weight climbing down stairs, as going down stairs is more jarring on the knees than walking down hill. Leg presses and squats are also good to do with your pack. For leg presses, find a tall stair or two or a low bench. Press up to stand on the bench with one leg, then bring the other leg up to meet it. Step down and repeat on the other side. Keep increasing weight and repetitions as you progress through your training.
Overall, if you can aim for at least one long training hike per week with a loaded pack (even better if you can find some place with a little elevation gain) you will be working the muscles you need to help get you to the top.