Walking is something that we do every day, without thinking about it. Its seems very simple and straight forward. You just put one foot in front of the other and you move. But it is kind of complicated. It has a lot to do with forces. Newton's laws of motion are involved. So what do forces have to do with walking?
Newton's third law states that every action has an equal and opposite reaction. This is relevant to walking because when you put your foot on the ground, you are applying a force to it. In doing this, the ground also actually applies an equal force onto your foot, in the opposite direction, pushing you forward.
When running, you tend to take longer strides because you are applying a larger force to the ground, so the ground must apply a larger force onto your body. When a larger force is applied to your foot, you go farther, taking a longer step.
Another law of motion that is at work is Newton's first law of motion. An object in motion stays moving, and an object at rests stays at rest unless an outside force acts on it. If people didn't apply forces to the ground, they obviously wouldn't move. This is because the ground then doesn't apply a force to the persons foot. They would stay at rest. But also, if it weren't for gravity, we would all float up into space the second we took one step. This is because the ground applies a force onto us, so we are set into motion. Gravity keeps this force from pushing us all the way into space.
(there is also centrifugal force that wants to fly people off of the planet, but ehh.)