Many race walkers do not hold their rear foot on the ground long enough. The longer you leave your rear foot on the ground, the more efficient your stride, for many reasons:
Your hips are able to pivot, thus lengthening the stride and allowing time for your leg to swing forward and your heel to strike the ground.
The motion stretches your hip muscles as they swing the leg forward, and the resulting reflex pulls the leg forward faster. As the faster-moving swing leg propels your body forward with greater force, you gain even more speed.
Your body exerts a force against the ground due to gravity. When you stand still, this force is completely vertical. By keeping the foot on the ground longer, the ground reactive force of the body’s weight becomes more horizontal than vertical when you lift your heel and move to toe off. This force helps maintain contact with the ground while contributing to forward body propulsion.