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.
This is just like cars that consume more fuel in stop and go traffic than when cruising down the highway. It wouldn't surprise me if it were a universal law.