Most people who backpack carry somewhere between one-sixth and one-third of their own body weight on their backs. Regardless of how much stuff you decide to carry, you'll have a much more comfortable hike if you increase your physical fitness before you ever set out.
Backpacking requires two types of fitness -- cardiovascular and muscular. Cardiovascular fitness is built through aerobic exercise, and muscular fitness is built through weight training. Some people choose to improve their fitness by walking on a treadmill or around town with a weighted backpack on their backs. But you can also prepare for a backpacking trip with more traditional exercise.
Cardiovascular fitness is achieved through aerobic exercise, including walking, running, biking and swimming. This exercise strengthens your lungs and heart, and will increase your body's ability to circulate oxygen. The more rigorous the hike, the more important it is to have a strong cardiovascular base.
Weight training will help to prepare your body for the strain of carrying a backpack on the trail. If you currently belong to a gym, ask a trainer to construct a workout for you that will prepare you for backpacking. If you're on your own, concentrate on exercises that strengthen your legs and back, and exercise each body part two to three times a week. Try bent rows, deadlifts, squats and pull-ups.
Your cardiovascular health is important, but your legs are your motor. They move you and support your weight and your pack's weight on the trail. For this reason, exercises that emphasize leg strength are essential. Try doing some squats, lunges, calf raises and step-ups. For additional strength training, choose a cardiovascular exercise like running, walking or a using a stair-climber to exercise your lower body.
After each exercise session, take a few minutes to gently stretch the muscles that you worked. And don't leave the stretching routine at home -- use it on the trail. At the end of a long day of hiking, take a few minutes to stretch your chest, which may be constricted from withstanding the weight of your backpack all day, your back, calves, both your quadriceps (the fronts of your thighs), and your hamstrings (the backs of your thighs).