There is a huge difference between treadmill running & outdoor running. I do both. Outdoor running uses a lot more resistance. You do not have handle bars to grab hold of either. It is also much tougher on the joints. I run 3 miles a day on the treadmill ~ work. On the weekend, I run the neighborhood & find my hips just ache terribly after pounding on the concrete.
If you do a search of this forum, there is another thread out there where they disscussed what settings a treadmill would need to be at to mimic actual outdoor running.

Good luck!