Lots of good suggestions so far, I will echo most, start off slow and build up. Hiking with a weighted pack is the best if you time your hikes with an app such as runkeeper and push yourself to beat your previous time. Basically racing against yourself. I would do that 3ish times a week, then do some form of upper body workout along with kettlebell swings to hit the legs. In my opinion you dont need to do crazy long hikes, for me 2-3 miles is the sweet spot if you are pushing yourself. It doesn't take forever to do, so it is enjoyable, not monotonous. Trail running is another great workout. Find a trail that will give you 1-2 miles of uphill and take off. Same as before time yourself and race against yourself.
Intensity is everything.