How fast can a people run?

Short distance sprint: about 12-15 MPH

Long distance run: about 5-8 MPH

Though this number is extremely variable by both age, sex, and fitness level. Men are typically taller than women, thus having longer legs and have broader chests for a larger lung capacity, as well, they don't have extra weight on their chest. Typically height, lung capacity and the size of breasts come into factor.

Your usage of average can vary depending on what you mean exactly, so both top speed sprinting, and paced long distance running has been listed to answer you question most comprehensively.