Technique is just as important as being in shape. If you are exhausted, you may be wasting energy. Read about and perfect the rest step, rythmic breathing and proper use of trekking poles. Also learn to properly shift your weight with each step. Shift weight over the forward foot and stand up - do not put the foot forward and pull your center of gravity forward. Avoid a lot of side-top-side wobbling. Step over obstacles instead on top of them. Try to place your foot horizontal on small features. Start slowly and gradually and steadily work up speed. Avoid stop and go - be like the turtle. Take a short (5-minute) rest stop every hour. Eat munchies at each stop, drink water.