While a lot of focus is placed on speedy animals such as cheetahs, falcons, and crocodiles, we are particularly remarkable because humans are the best continuous long distance runners in the animal kingdom*.
Truth is that our raw speed is rather unremarkable when compared with the significance and uniqueness of our relative speed. 
There are several characteristics that make humans uniquely suited to long distance running, including bipedalism and sweating. With the ability to run on two feet rather than four, humans where able to travel while consuming fewer calories—a critical resource that was often in short supply. Also, unlike many animals that released heat through panting, humans had particularly advanced sweat glands and could cool themselves while continuing to travel. This meant that humans could keep running at a point that other animals would overheat. There is even the possibility that, with free hands, humans can carry water to rehydrate while running.
Fossil evidence of these features suggests that endurance running is a derived capability of the genus Homo, originating about 2 million years ago, and may have been instrumental in the evolution of the human body form. 
This ability to run long distances gave humans an evolutionary edge through an early technique called “persistence hunting.” In hot environments most animals rested in the heat of the day. Surprisingly, such limitations made this time of day ideal for humans to hunt. Humans lack big teeth, claws, horns, and speed. Yet a human can out-travel an antelope. While the faster prey might be able to temporarily outrun the human hunters, humans could also exersize another advantage, their advanced brains, to track animals, even when they could not directly see, hear, or smell the prey. Thus humans would continue to run down the animal until it eventually overheated and collapsed. Such hunts can last an average of two to five hours of continuous activity.
This capability was one of the core advantages that contributed to the human race not only surviving but thriving in harsh environments.
the full picture of why we became human must include running, it absolutely must.
* Granted, dogs can travel further in cold climates, but in most areas, dogs cannot maintained the same continuous locomotion as humans and eventually must stop to pant and relieve heat.
1 Bramble, Dennis M., and Daniel E. Lieberman. “Endurance Running and the Evolution of Homo.” Nature 432 (2004): 345-52.
2 Fleming, David. “Slow and Steady Wins The planet.” ESPN.com. ESPN the Magazine, 11 Feb. 2011. Web. 15 Sept. 2013.
Endurance Running [Wikipedia]
Persistence Hunting [Wikipedia]
The Human Body Is Built for Distance [New York Times]
Fair Chase [Outside Magazine]
persistence Running [BBC] – video clip of San bushmen running down a kudu
Daniel E. Lieberman [Harvard] – Home page for Daniel Liebermen, one of the top researchers in this field
Did Humans Evolve to Be Long-Distance Runners? [blog] – examples of challenges to the endurance running hypothesis