(or you can define it this way...)  
to go quickly by moving the legs more rapidly than at a walk and in such a manner that for an instant in each step all or both feet are off the ground.
“Running to him was real; the way he did it the realest thing he knew. It was all joy and woe, hard as a diamond; it made him weary beyond comprehension. But it also made him free.” ~ John Parker, Once A Runner
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Once A Runner by John L. Parker Jr.

(The runner's Bible)