Take longer strides. It's a body-friendly way to increase speed. Many runners try to go faster by taking more steps more quickly, but this is tougher on both the knees and the lower back. "Find the stride length that enables you to be lightest on your feet and moves you fastest," says Jay Blahnik, a fitness consultant for Nike who also trains elite-level runners and speed walkers in Laguna Beach, California. "Test out several strides. Once you find the one that makes you feel like you're gliding rather than pounding, that's your ideal length."