To lose weight, which is more important, how long I exercise or how hard?

At the end of the day, weight reduction is a direct result of how many calories you consume versus how many you burn. You can think of it like a checking account if you wanted to spend all your money and get a balance of zero. In order to accomplish this, each day you have to spend more than what is deposited into your account.  The same is true for weight reduction.  You want to consume fewer calories than you burn in order to hit your goal.

The answer to the question, “which is more important, how long or hard you exercise”, is it depends.  It depends on how many calories each of those cardio sessions burn.  The harder you work, will inevitably allow you to burn more calories in a shorter amount of time, however the increase in intensity will likely prevent you from being able to sustain the activity for any significant amount of time. The reverse is true for long duration exercise. In order to exercise for extended periods of time, you will not be able to sustain higher intensity levels. With the lower intensity levels, you will not be burning many calories at any given time. It lacks efficiency. Long duration activities will, however, allow you to burn more calories simply due to the fact that you can continue the exercise for quite some time without getting completely fatigued.

The above two points serve as a great opportunity to meld them together into what is commonly known as interval training. In interval training, you maximize your calorie efficiency by exercising at high intensity levels for short durations, and then recover at lower intensity levels for longer durations. This creates an ideal form of exercise that truly maximizes caloric burn, while offering you rest and ensuring that you are able to fully complete the workout. 

The most important thing to remember when doing cardiovascular exercise for weight loss is to increase your metabolism. It is well known that weight training will increase muscle which will aid in caloric expenditure because of the increase in metabolism. We now have to bring the weight training concept into your cardiovascular program. To increase your metabolism through cardio you must overload the body. The best way to overload the body is by doing interval training, which are short periods of high intensity work followed by recovery periods. When introducing intervals to your program it is best to begin with 30 second high intensity periods (high intensity could be a fast walk or for more advanced individuals, a sprint) followed by a 30 second recovery (this is a walk to a light jog, depending on your fitness level). You can start with three intervals, or a total of 3 minutes, before recovering to an easy walk for 5 minutes. Once you have completely recovered, repeat the intervals. Doing this pattern for 30 minutes several times a week will help boost your metabolism and will burn a lot of calories both during the workout and after you finish. The rest of the week should be a lower intensity, steady-state training to help burn additional calories and allow your body to recover and prepare for the high intensity days.