P90X is great. As others have pointed out, doing training on a treadmill or step-mill is great, but it is definitely not sufficient by itself. The stabilizing and supporting muscles and tendons MUST be worked out too. Not only will this allow you to go faster and farther, but it greatly reduces the chance of injury.
I do trail running, cycling, racquetball, and weights to train when I can't be out backpacking.
Another thing that I do is to wear a pack loaded with 35# of textbooks while mowing my lawn (push mower). I have a very hilly yard and it takes about an hour and a half to mow. I don't use the self-propel feature and find that up and down the hills pushing the mower with a heavy pack makes for an excellent work out. Plus, there's lots of up and down, turning, start/stop, and off camber walking which hits many of those stabilizer muscles too. I'd be mowing the lawn anyway, so I might as well get some training in at the same time. Neighbors think I'm a bit weird, but whatever.