Start out walking. I agree that running will get your heart rate up but if your out of shape stick to walking. Before I go out on a 3 day hike, I'll start training for it a couple of months ahead of time. I put on a walkman with some tunes that have an uptempo beat that I can pace myself to. Also it makes walking way less boring. I can do about 3 miles on 1 cd starting out. I build to 7 or 8 miles non-stop. When I reach that point I kinda start over on the mileage only now with a 20 lb pack. Increase the weight and the distance until you are confident you won't bonk. When I do reach the trail. I pace myself, keep myself hydrated and enjoy the view. If your just starting out, don't burn yourself out trying to acheive the experience.