We all have seen elite endurance athletes run at a superbly fast clip for almost forever. Crossfitters who can perform really well for several metcons over 3-4 consecutive days. MMA fighters who can fight at high intensities for 15-25 minutes. There is a secret to building this large work capacity other than just high intensity work.

Build a huge and efficient aerobic system.

You build your aerobic system more efficiently by exercising at lower intensities. However, many athletes are not going slow enough and thus not able to build their aerobic engine efficiently. Over a long period of time, an underdeveloped aerobic system can cause plateau in fitness. On the extreme end, it can lead to burnout or overtraining. 
The aerobic system, unlike the anaerobic system, has a greater room for development and thus, a properly developed aerobic system will be able to bring your performance to another level in most of the sports.
So how slow do you need to go?

If you are measuring your intensity using the 5HR Zones, then your aerobic efforts should be spent in Zone 2.
If you are measuring your intensity based on a rate of perceived effort scale of 10, then it should be hovering around 6-7. 

In short, the pace should be conversational (it should not allow you to sing though). Another way to find out if you are going slow enough is to use the breathing test; you should be running at a pace slow enough to run to breathe in and out through your nose without using your mouth comfortably.

The heart is a dumb muscle; as long as you stay in this heart rate zone/ conversational effort for a sustained period of time, it really doesn’t matter what form of exercise you do – good news for athletes as they can apply this concept and introduce sport specific exercises to develop the aerobic system to the specific requirement of the sport.

If you truly want to build a large engine which helps to increase sports performance, do cater time to put in those easy continuous efforts!!! You might feel that you are not doing much as these sessions might feel “guiltily easy”, but the adaptations gained are worth it over the long term!!