You might think that getting a few miles in on the treadmill is just as good as a run outside, but you would be sorely mistaken. Running on a treadmill doesn't provide the same workout, for many reasons.
Due to a lack of wind resistance and many other factors, a treadmill does not require as much energy as running outside. You will actually burn more calories running on a trail or road than on a treadmill. Treadmills partially propel you forward, unlike the unyielding flat ground outdoors. If you have to train on a treadmill, put the incline at 1% to make your workout more productive.