According to new research, neither the slow but steady tortoise pace or a fast constant run is the best workout. The study showed that interval training produced much higher results than either a long walk or constant running on a treadmill.
Interval training consists of stretching first followed by a warmup period of only walking. Then going straight from the walking speed to an all out sprint. The sprint should last long enough for you to start running out of air. If you are pushing yourself hard enough this should take somewhere between 30 seconds and a couple of minutes depending on your current condition.
After the sprint, you slow back down to a walk speed. You should walk for quite a bit longer than you sprinted, however no so long that your heart rate that was pushed really high while sprinting comes back down to that of a normal walk. Then you repeat the sprint/walk portion until your workout is completed. It is warned that because you are keeping your heartrate higher than normal that this is not for those with heart risk, and it's also easy to over do it. Start small and work up.
I would like to note that this study was done on a very small group, so it's results may prove to be inaccurate. Their initial findings however warrented a post from me. Hit the link to read about the study.