While treadmilling may well be the closest exercise to running there is, it isn't actually running. Why? The main thing I see is that, given one's body has no momentum when treadmilling, the stride, especially for inexperienced runners, is often dramatically different than the same individual's stride when running on a stationary surface. The main stide differences I observe among those I coach are shoulder carriage, arch of the back, and worst of all, over striding (i.e. the foot strike is too far out in front of the runner's hips). One of the new runners on my company running team was complaining to me just this afternoon about back and knee pain when she treadmills. I videoed her with my phone both outside in the parking lot and inside on the treadmill and she was stunned at how different her stride was. "That's it," she said, "I'm running outside from now on, to hell with the heat."
Fat old man PRs:
1-mile (point to point, gravity assist): 5:50
5K (gravity assist last mile): 21:31
10K (first 10K of my Half Marathon): 48:16
10-Mile (first 10 miles of my Half Marathon): 1:17:40