I find its much more difficult for me to run on the treadmill. I can comfortably run an 8:30/mile pace outdoors. When the roads are really bad, I go to the gym and run on a treadmill. I feel like I'm sprinting doing the 8:30/mile pace and its terrifying--I feel like my feet are going to fly out from underneath me. I have to slow my pace significantly to feel comfortable.