Another problem is not doing it enough. Walking a couple miles once a week isn't going to do much. You have to make it a habit, part of your routine, and do it every day.
Also not only is your body very efficient at walking/running (losing 10lbs of fat requires an average person to run from SF to LA) there's evidence of a constrained total energy expenditure model. If you try and create a large caloric deficit through exercise you become more efficient at the exercise (so each incremental step costs less calories) and your metabolism slows down (and your NEAT - non exercise activity thermogenesis - levels drop) to conserve energy for you to expend on exercise.
