At a clip of 7.5 minutes per mile, you'll do two miles in 15 minutes. It seems counterintuitive. Running feels and looks harder. The body is moving faster, getting hot and sweaty. A case could be made that it's doing less, that running is more efficient.
Runners train themselves to move their bodies economically.
They develop a swing and a rhythm and cover more ground than walkers with each stride. Once they get their second wind, they hit cruising speed, seemingly moving without effort, benefiting from the power of inertia. Think of a locomotive pulling a long line of freight cars. It works hardest when it must get everything moving from a standstill. At 10 or 15 miles per hour, it's still pulling a load.