There are a wide variety of approaches to training for aerobic performance. The optimal approach requires varying the duration and intensity of your workouts. Many people mistakenly train at the same moderate intensity during every session.
The most effective approach is to perform a large percentage of your training at low intensity, sometimes called long distance, slow training. This is combined with some moderate intensity training and some high intensity training such as sprint and interval training. The level of intensity and duration of training should be varied from week-to-week and month-to-month. Designing training plans with these variations is called periodization.
SERIOUS Training for Endurance Athletes
A good resource for endurance training plans is the excellent book, SERIOUS Training for Endurance Athletes, written by Rob Sleamaker and Ray Browning. S.E.R.I.O.U.S. is an acronym for the components of their training plans: Speed, Endurance, Race/Pace, Intervals, Overdistance, Uphill, and Strength. If you use this book, you might find this Microsoft Excel spreadsheet useful:
A common misunderstanding is to think that the more training, the better (A version of "No Pain, No Gain"). It is important to remember that training works by inducing injury to your body. The improvements to your body that allow better performance occur while you are resting and recovering from the training induced injury. Training too much can lead to decreased performance, illness, and injury.