[x-factor]

Bear in mind, there's a difference between sprinting form and distance form. When you run slower, you're not leaning forward, and you're not going to strike with a different part of your foot. Point being, you're right that you shouldn't be reaching with your heels, but neither should you be jogging on the balls of your feet as if you were sprinting.

To develop a feel the difference, try running about 200-400 meters smoothly throttling up from a walk to a full sprint and then back down. You should feel your center of gravity, your body lean, and your foot strike move forward and back as you change speed.

If you want to do something to strengthen your joints. I'd suggest you think about taking up two-man beach volleyball (assuming there's a decent place to play near you, by that I mean somewhere with guys who play at a high tempo). Explosive movements in multiple, unanticipated directions in soft sand is the best way I know to strengthen joints and improve explosive movement. As a bonus it will be good for your hand-eye coordination and a good way to meet chicks.