[istari]

This is something I always wondered about.

I get FAR more tired running 2 real miles than 2 "miles" on a treadmill.

On a treadmill, both of your feet are in the air for a significant amount of time, and you're not being carried backwards. Yet, I bet that distance still gets counted.

[jongraehl]

The only significant differences are constrained stride length, and wind resistance (which definitely matters even before 10mph).