Sane & Effective Cardio:
Routine A (moderate pace): Walk at 4.0 miles/hr, and you will cover around 1.3 miles in 20 minutes. Next time you come in to do routine A you should try 4.2 miles/hr and cover a little more distance.
Routine B (challenging pace): Walk at 4.0 miles/hr, but raise the treadmill to a 5% incline. Next time increase your speed but keep the incline the same. When you reach the fastest speed you can handle without jogging, increase the incline a little more each workout.
Routine C (strenuous pace): Walk at your top speed at the highest incline you can endure. If you find it extremely hard to maintain such intensity then do intervals. That is, 2 minutes at the top speed and top incline followed by one minute at a lower speed and lower incline.
TREADMILL EXAMPLE 2:
Routine A (moderate pace): Walk at 4.0 miles/hr, but raise the treadmill to a 5% incline. Next time increase your speed but keep the incline the same. When you reach the fastest speed you can handle without jogging, increase the incline a little more each workout.
Routine B (challenging pace): Walk at your top speed at the highest incline you can endure. If you find it extremely hard to maintain such intensity, then do intervals-two minutes at the top speed and top incline followed by 1 minute at a lower speed and lower incline.
Routine C (strenuous pace): Time to start Jogging. If you jog at 5.5 mph you will cover 1.8 miles in 20 minutes. Your goal is to keep increasing the mileage within those 20 minutes. So next time do 5.7 mph or do intervals-start at 5.5 mph for 2 minutes then go up to 6.0 mph for 1 minute, and then back to 5.5 mph for 2 minutes and repeat.
Once you become better at this, increase the speed of both the 2-minute "moderate" interval and the 1-minute "hard" interval. If you really want push to push yourself after a few weeks of the program, keep the intervals to 1 minute moderate and 1 minute hard for the whole 20 minutes.