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 endurance training at low intensity, sometimes called long distance, slow training. This is combined with some moderate intensity and high intensity training such as sprint and interval training.
More recently, some have advocated scaling down or almost completely eliminating moderate intensity training. This type of so-called polarized training consists of a large component of low intensity endurance training, typically 80%, combined with some high intensity training and little or no moderate intensity training. This is sometimes also called 80/20 training.
The level of intensity and duration of training should be varied from day-to-day, week-to-week, and month-to-month. Using training plans with these variations is called periodization.
Two good resources for endurance training knowledge are the excellent books, Training for the New Alpinism: A Manual for the Climber as Athlete by Steve House and Scott Johnston and Training for the Uphill Athlete: A Manual for Mountain Runners and Ski Mountaineers by Steve House, Scott Johnston, and Kilian Jornet.
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.