A team of pathophysiologists at the University of Milan has found that climbing stairs or walking for short bursts allows people to consume 20% to 60% more energy than if they do the same activity nonstop for the same distance. In their study, published in the Proceedings of the Royal Society B, the...
3 miles in a treadmill does not feel like 3 miles on a track or park.
Try it yourself. There's different muscles in my feet that get tired. I'm sure a treadmill is better than no training at all, but if you are aiming for a 5k try to do real walking or running.