It’s that old hiker’s saying: “A pound on your feet equals five pounds on your back.” Many who are first thinking of backpacking consider the weight on their back as the most difficult aspect of a trip. And while a backpack itself may weigh more than shoes, the energy output expended per pound of shoe is much higher than for a backpack. Walking remains the most popular aerobic activity in the United States. More than 145 million adults include walking as part of their physically active lifestyle, with about six to ten adults having reported walking for at least 10…