One of the biggest problems with cardio is complacency. Doing 20 minutes of intervals at the same speed or intensity over and over will not garner the best results. For this program, you need to go beyond that base level.
When you push the treadmill faster or increase the incline, you challenge your body with new stimulus. Interval training is difficult as it is, and but realize that the harder you push, the greater effects the training will have.
You'll be amazed by what your body can do when it's challenged. You know you are stronger than you think, but—believe it or not—you can run harder and faster for longer than you think!