Duration refers to time involved exercising. ACSM recommends doing aerobic exercise twice weekly, a minimum of 20 minutes or more each session. I concur with ACSM's guidelines encouraging and challenge each of you to meet or even surpass them in a safe and effective manner. Not focusing on cardiovascular conditioning allows us to greatly vary the intensity level of our extended exercise. Exercise intensity must also be addressed.
Intensity refers to effort put forward and the energy expended during the exercise.
Maximal forms of effort will result in minimal duration. You will get tired quickly. Therefore, to burn off those few extra calories, plan to extend the exercise within your duration limits by emitting only an average to above average effort or intensity level. Again, do not get this confused with cardiovascular training which aims to train the heart and lungs. We are merely burning extra calories.