There are a thousand and one ways to work out on a treadmill, but my favorite is intervals. Changing your pace every few minutes busts through the boredom often associated with indoor running, and once you set your pace on the treadmill it's difficult to cheat. In this workout the set speed pushes you beyond my comfort zone with a gradual build, and the minute recovery is just long enough to lower your heart rate to rev myself for the next interval. Another reason to love intervals: they help you lose belly fat. This workout combines running with walking, so if you new to intervals you should give it a try.
As always, feel free to adjust the speed up or down as you feel necessary; just make sure you're challenging yourself. If this workout doesn't float your boat.