You'll get equally fit running on a treadmill or running outdoors. In fact, many distance-running athletes use the treadmill for training to save their legs from the pounding of roadwork. However, there is a slight difference in energy expenditure (calories burned) between the two.  Outdoor running burns slightly more calories than treadmill running at the same speed due to lack of air resistance on the treadmill. Researchers studying this phenomenon found that setting the treadmill at 1% elevation pretty much equals things out.  So set the treadmill at 1% so that treadmill walking or running mimics outdoor exercise.