Well here's my two cents worth.

Do some long walks with a pack and 20 lbs in it. Don't haul butt but try to keep a steady and fairly quick pace to get your heatr rate going and up there. Make sure you've got a decent pair of shoes too so you don't have to worry about shin splints. Also add in some hills if you can after a few walks at least. Up the weight at your own pace. Pushing the pace and distance at your own leisure won't hurt either.

If you can't get out and walk, try and do some running. At least a few miles a day once you get your endurance up. If you have access to a gym or some sort of weights and other exercise equipment, take advantage of that. You'll want to work on core strength and leg strength too. Things like leg presses and squats will help the legs out. Nothing too heavy of course if you're just starting out. I would say at least a weight slightly exceeding your body weight at first and then upping it in time to try and simulate pack weight.

As far as the core strength goes for me, sit ups obviously are going to work abdominals. Also for the abdominal area, if the gym has it, there is a thing (no idea what it is called) where you hold yourself up with your forearms paralell with the ground and bring your legs up to a 90 degree angle with the knees bent. For back, try doing some rowing excersises and another thing (again another thing where I don't know the name), but it is an adjustable object that leans at a 45 degree angle and you stand in it. It allows you to bend over the pad facing the ground and then coming back up to the starting position. If you can find what it is I'm talking about, use that and just start out with your body weight at first, adding weight when you're comfortable. You'll get sore but after a while you'll up the weight and the soreness will pass. When you add weight, use free weights and hold them with your arms over your head with the weight resting at the top of your back at the base of your neck. Also try doing side crunches with this same thing as well, it helps. For me right now, I'm doing 35 lbs each way without soreness, which means my pack will not bother me on hikes at all. Some of this stuff is really hard for me to explain without some sort of visual, so I hope you get the idea or maybe see someone doing the same thing and realize it.

As for cardio at the gym, I love the hell out of the elliptical. No impact on the knees and if you get the speed going for 30 straight minutes or so you'll get a decent cardio workout from it.

Hopefully this stuff kind of gives you some help.