Everyone else gave some good tips as far as doing some walking. I would also recommend cross training (running, cycling, inline skating or whatever other activity you are able to do). One thing no one else mentioned is doing some lighter "cardio" weight lifting. You mentioned that you are a weight lifter with a bulky build. One way to tone that down a little is to do a lot of reps with lighter weight. This will also bring up your heart rate, and build your cardio. I used to lift weights to gain mass (although I never really did...at 6'5" I found that it is tough to build mass), and after a while I decided to do lower weight and higher reps. This is a good way to do what you like (lift weights), and build your cardio at the same time. It should also help you lose some of that bulk that is going to be tough to carry on a long hike.