What is the best way to regain fitness using a treadmill?

I have to get my fitness going again, and was wondering what is the best way using a treadmill. Like, should i do long periods of time on a lower speed, or high speeds for a shorter time? etc.|||Actually a bit of both. Start out by flat walking. Then go to inclined walking then to flat running to inclined running. Set the speeds so that you feel challenged. 20-40 minutes a day should really do it.