Here is a workout to do if you are interested in an advanced HIIT workout (high intensity interval workout). This workout is meant to be done on the treadmill.

20 minute workout:

2min at 6 mph
1min at 8mph
1min at 9mph

2min at 6mph
1min at 9mph
1min at 10mph

2min at 6mph
1min at 8mph
1min at 10mph

2min at 6mph
1min at 9mph
1min at 10mph

2min at 6mph
1min at 9mph
1min at 12mph

If you would like to make this workout harder, increase the incline to a 2 or 3.