+1000

When you're running on a treadmill, you're indoors. There's no wind, cold temp, rain, and you don't have to be bundled up....

Also the "ground" is moving beneath you like others said before... That means you can hop half a mile if you wanted to... or take long strides so that the machine "works" for you.... Runners never run exactly 10.000 mph when they're outside... it's impossible, pace varies as fatigue and grade changes... But on a treadmill it's always done... just set the speed...