How to decrease stress on your joints and run faster.
One way to decrease the stress on your joints while running is to contact the ground with the ball of the foot. This allows the muscles and tendons in the calf to absorb some of the force and reduce the stress on the joints. If you are not strong enough to run like this, just run intervals and gradually build your foot and calf strength. Be careful that you are not reaching ahead and contacting on the ball of the foot ahead of your body. Also understand that the heel does not need to be high. The foot can land almost flat: either slightly off the ground, or touching the ground with very little weight on it.
Correcting the location of ground contact is another way to reduce stress on the joints. The ball of the foot should contact the ground under the hips rather than in front of the body. If the foot lands far enough ahead of the body, braking will occur in a variety of ways. Braking occurs when the foot is not moving backwards at ground contact. An extreme form of braking is when the foot is actually still moving forward when it contacts the ground, usually contacting with the heel. This creates the greatest joint strain because the heel contact (rather than the ball of the foot) carries force through the bones and joints. In addition to this skeletal connection, the force is greater because the foot has a velocity opposite that of the ground: if the foot is moving forward at 5 mph and the ground is "moving" the opposite direction at 10 mph, then the net impact is like an impact at 15 mph. It seems that to achieve this type of braking, the legs must be passively waiting to contact the ground rather than actively pulling with the glutes. To correct the braking, just activate the glutes before ground contact accelerating the foot backwards before contacting the ground under your hips (or slightly ahead of the hips).
Another form of braking that I have observed appears to occur when people are trying to stay on the ball of the foot, but they are also reaching ahead and contacting in front of the body. They usually land with the knee bent more than is necessary and have a lot of vertical movement rather than smooth horizontal motion. This probably uses more quadriceps than glutes which contributes to the vertical motion. Again, the remedy is to activate the glutes to pull the foot under the body rather than leaving the foot to passively wait for the ground.
Both of these techniques, contacting with the ball of the foot and contacting under the hips, help you run faster. They complement each other, and allow the foot to pull on the ground to increase horizontal force backward which moves the body forward.