I agree that putting on a pack and hiking is the best training you can do. But it shouldn't be the only thing you do. Including some other forms of training as well will give better results than hiking alone.
Firstly, strength training should be part of your overall training program. Leave 48 hours between ST sessions (which implies 3 times per week). Great moves for hikers include:
* squats (works the quads and glutes, and are awesome for powering up steep slopes)
* planks (works most of the core muscles, and build back strength for supporting your pack all day)
* calf raises - do them barefoot to give your feet and ankles more of a workout, as well as just the calves, which helps you to stay balanced on a rough and uneven trail. If you can graduate to single leg calf raises, this really helps the balance aspect
Secondly, do some High Intensity Interval Training. Sprint for 30 seconds, then recover at your normal pace for 90 seconds. Repeat. Sprinting doesn't have to mean running (although that is probably the best form given the context of hiking) - you can 'sprint' on a bike, or an elliptical, or whatever. 1 or 2 HIIT sessions per week is plenty, and because you can probably maintain that intensity for only 10-20 minutes at a time, they are a very time-efficient workout
HIIT is absolutely the best way to increase your VO2Max - which is important to hikers for 2 reasons:
1. It will allow you to go up steeper slopes faster
2. A higher V02Max allows your body to get more of its energy from fat, rather than carbs. As fat has 9 calories per gram, and carbs and protein just 4, this will allow you to lighten your food load.