Running is by definition the fastest means for an animal to move on foot.
It is defined in sporting terms as a gait in which at some point all feet are off the ground at the same time.
It is a form of both anaerobic exercise and aerobic exercise.
Running is a complex, coordinated process which involves the entire body.
Leaning forward places a runner's center of mass on the front part of the foot, which avoids landing on the heel and facilitates the use of the spring mechanism of the foot.
It also makes it easier for the runner to avoid landing the foot in front of the center of mass and the resultant braking effect.
There are many injuries associated with running (due to it being a high impact activity).