Training endurance ends up maintaining or increasing the total capacity of the nutrition delivery system. The whole body is moving efficiently, meaning that all the endurance or posture systems are involved. These systems working at once put a drain on the oxygen part of the nutrition delivery system. After training, that system grows primarily in volume. Distance running is the ideal way to train endurance, although it is important enough that if anyone cannot run it can be trained in a number of ways.
The type of processing that occurs during running cannot really be achieved with anything else. The brain is scanning, at the same time the serotonin system is activated, making the mind think more idealistic as it scans.
Ordinarily, endurance is performed twice a week, gradually increasing the amount of time, keeping the intensity at the point where belly/side breathing and not chest breathing is occurring.
One of many examples of why each of the intensities have to be trained has to do with the increased capacity that develops with endurance training. If your energy is low, it means usually that excitability decreased due to deconditioning. After improving your endurance, you will notice that now when you do have some stimulant you have more energy. If you trained only your endurance system even more, you might be at the point that the actual stimulant makes you feel overwhelmed because the nutrition delivery system is now able to deliver sugar much faster. In other words, instead of alternating between groggy and overwhelmed, if you are more consistent with your training you could have energy all the time.