I think that also comes down to not being able to make the subtle adjustments to your pace you do when you run outside. Being able to slow down or speed up without thinking about, even just for a few seconds, really makes a difference.
It's also by far the most TEDIOUS form of exercise. I love running and go out on 4 or 5 runs a week. Occasionally I have to do my run on a treadmill, and I lose the will to live after about 4K. Even if I'm listening to music or watching TV I find it very hard to be motivated.
If you get a...