Newton's second law of motion states that: "The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object." Meaning that the runner will remain at rest until a force is acted upon it. The runner won't move until he/she pushes there foot on the ground to go forward.
Newtons third law: "For every action, there is an equal and opposite reaction." the runner as an equal and opposite reaction when he pushes his foot off the ground to start running. Meaning that when he pushes back on the ground he accelerates forward while what he used to push off of pushes in the opposite direction with equal amount of force.
How much your mass is determines how much force you need to use to push off at the same speed as someone else who has less mass.