It depends what your goals are. You could go faster or up a harder incline and you could work out less time - using more effort during that time. You could also do the treadmill at night - but if you always do the same thing on it then your body will adapt to be as efficient as possible when doing that activity. In other words, you'll burn less calories the more times you do the same exercise.

This is why I'm running faster, doing more inclines, doing different types of exercise, and always increasing the weights I lift. The body adapts quickly so you have to keep "tricking" it.