Each leg has to launch you forward with each step. Each leg then has to perform a rapid-fire braking motion when you land to keep you from falling facedown.
The key to all this is a powerful quadriceps, but strong quads are not your birthright as a runner. You need to fortify them through strength training.
Best exercises: Head straight for the leg-press machine, but only do shallow squats to spare the knees. Also, do your leg presses one leg at a time (just dangle the resting leg off to the side) to ensure that you're not perpetuating any strength imbalances.
You can do another great quad-builder on the treadmill, but be careful with this exercise. (Have a friend spot you the first few times.)
To perform the exercise, set the machine at a 15-degree incline and a steady walking pace. Then quickly turn around and walk backward. Be sure to use the handrails if they're available. Your goal is to be able to walk quickly but in control. Try doing this for two to three minutes at first, and gradually work up to five to 10 minutes.
Shop for strength-training gear at the Active Sports Mecca