When training for my last Whitney hike, on top of lots of day hiking and free weights, I monopolized the Stairmaster at my gym. I did 45 minutes on progressively harder settings 3 times a week. Let me tell ya, I bounced up that trail! In fact, I summited twice, but that's another story (on my blog if you're interested).

It helps to tailor your training to the kind of hike you're doing. If you're going to drag a huge pack relatively short distances, then your weight training is probably going to help you a lot. If you want to go light and hike 30 miles a day, then you need endurance training.