Running on uneven ground will help more than running a flat track will. In either case, you need to run as hard as you can as far as you can without injury. Warming up and stretching are critical to both speed and distance. "Bouncing" during any warmup is taboo and often taught incorrectly as a part of stretching. Do NOT bounce. It is far more dangerous than running cold. (As a distance runner, warmups were more about stretching on a gradual basis and not all at once. To stretch and warm up the muscles took about 30 min). Sprinting like EOD has suggested is a very good way, but only after warming up and stretching.