The following workouts can be performed on any type of cardiovascular machine (treadmill, elliptical, recumbent bike, etc.)

20 Minute Workout
Exercises at a slow, comfortable pace for 2 minutes (Warm up)

Exercise as fast as you can for 2 minutes
Return to a slow, comfortable pace for 2 minutes

REPEAT this 3 more times

Exercise at a slow, comfortable pace for an additional 2 minutes (Cool down)

30 Minute Workout
Exercise at a slow, comfortable pace for 2 minutes (Warm up)

Increase to a brisk pace for 25 minutes

Return to a slow, comfortable pace for 3 minutes (Cool down)

40 Minute Workout
Exercise at a slow, comfortable pace for 3 minutes (Warm up)

Increase to a brisk pace for 7 minutes
Return to a slow, comfortable pace for 3 minutes

REPEAT this 2 more times

Exercise at a slow, comfortable pace for an additional 7 minutes (Cool down)

Any Length Workout
Exercise at a slow, comfortable pace for 3 minutes (Warm up)

Exercise as fast as you can for 1 minute
Return to a slow, comfortable pace for 1 minute

or

Exercise as fast as you can for 30 seconds
Return to a slow, comfortable pace for 1 minute and 30 seconds

REPEAT this as many times as possible

Return to a slow, comfortable pace for 3 minutes (Cool down)