Most police departments will tell potential recruits to practice running on real terrain instead of on a treadmill. This is because running on terra firma is much different than running on the treadmill. I have a treadmill and it is much easier to run on than real ground.

I would say that with the treadmill you get barely half of the real work out you would get from running on a trail. Granted, running on a treadmill does have its advantages. You can run indoors in your pajamas and in all weather conditions since you are oblivious to outside weather. With a treadmill you're less likely to be assaulted by a neighbor's dog or to be hit by a car.

Treadmills are good for recuperation and for general exercise. Real running should take place for more serious exercise and of course for training for marathons. Runners can use treadmills and trails together to achieve their optimum running state.