Surely because we never do any long sessions of running, CrossFitters can’t have any endurance or stamina… Wrong! The widely touted measure of aerobic stamina and endurance is VO2 max, the amount of oxygen that your body can consume during sustained aerobic exercise — medium intensity stuff like distance running or biking. There is a common misconception that long bouts of aerobic exercise are the only way to build stamina and endurance, but this simply isn’t true.
In the same way muscles must be taken beyond their limits and torn to stimulate growth, your ability to process oxygen must be pushed out of equilibrium in order to increase your capacity to process it. Running at some level below your aerobic threshold (i.e. less than 100% VO2 max) will do little if anything to increase this ability. To push it out of equilibrium and induce growth you have to workout at 200% or more of your aerobic ability. In metabolic terms, you need to work the high-intensity phosphagenic and glycolytic pathways in order to improve the medium-low intensity aerobic pathway.
Still want to learn more? Check out this article from the CrossFit Journal.