I've hunted Colorado a few years for elk and the first year I suffered from a constant nagging headache from taking on too much exertion too fast. So now I do the following:

-Currently into walking farm roads that have hills and walk the first 1.5 miles at a fast pace to loosen my legs and then the 2nd 1.5 miles I run 3/4 sprints of telephone poles. Run one...walk one. I haven't run in a long time so I'm working up to it. Don't do too much at the beginning as you'll only end up hurting yourself and will be unable to continue.
-Tonite I start my summer hockey league (Once a week) and it's a great workout.
-Get on the stairmaster as it is a great simulation of climbing.
-Ride bike if you have one in order to not get bored with the same workout or put the rollerblades on. Varying exercise might help you stay with it.
-Treadmill, if it has an incline, start out with a slight incline and work up from there. Put on a pack and walk with the incline which I've found to be a really good workout and is the closest thing you can find to walking uphill with a pack on.
-Drink as much water as you can handle in the months before and continue drinking as much on the trip as you can. I also take an aspirin or two daily as I've been told it thins the blood out. Not sure if it helps but I haven't had any headaches since putting a little effort into it.