I do regular, outdoor walking, not treadmill walking, so there are some differences.
If you can get yourself to walk at a speed that feels more comfortable running/jogging (usually approaching 5 mph), it is supposed to burn more calories than running/jogging that same speed.
I am working on getting a 5 mph average on my outdoor walks---I did it a couple of times, but mostly have been making 4.9 mph. I have walked on a treadmill on rare occasions--when staying in hotels--and I can't get that speed, because it's an average: sometimes going 4.5, sometimes going 5.5, and anywhere in between, and the treadmill just goes at a steady pace. (I hate treadmills and machine cardio in general!)
I would say, vary your workouts. Do some intervals with 'hills' and/or faster 'sprints'. Do some longer, steady-state workouts. Cardio Coach (which has already been mentioned) is a great idea, and made for the treadmill. Tracey Staehle also has a "Walking Strong" workout that is pretty good.