Walking is the most popular form of exercise among Americans.  That's according to a new survey conducted by the Sports and Fitness Industry Association.  Almost 80-million people say they walk for fitness purposes at least 50-plus times per year, more than any other type of activity.  Almost 30-million jog or walk regularly to get their exercise in, while 26 million others stretch and use a treadmill.  Lifting weights is popular as well.  Over 20-million use weightlifting machines to get in shape, and even more lift hand weights under 15 pounds.  Some types of sports are also being used to get cardio.  Fishing and basketball also rank among the top 10 most popular ways to exercise.