Why is there so much anaerobic, sprint and speed work if I am going to run in an endurance event?

Anaerobic training—sprint, or speed work as it is often referred to—is commonly and incorrectly devalued by its supplemental place at the end of many traditional training programs. It has the following benefits and drawbacks:

Benefits Drawbacks
Increases cardiovascular function Intensity hastens overtraining
Decreases body fat
Increases muscle mass
Increases strength
Increases power
Increases speed
Increases anaerobic capacity

Anaerobic training combines training in all three metabolic systems—the Adenosine Tri-Phosphate / Creatine Phosphate system, the glycolitic, or lactic acid system, and the aerobic, or oxidative system—through various methods stressing one or multiple systems at once. The time duration of individual efforts combined with the rest periods between efforts determines the metabolic system(s) stressed. These systems overlap and “feed” into each other, and as you train all three energy systems you are simultaneously training your aerobic engine (Fig 1.1).

The aerobic adaptations caused by anaerobic training are similar to the adaptations of high-volume endurance training. However, these adaptations come at much lower and safer training volumes.

Fig 1.1 (Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans. J Physiol. 2008 Jan 1;586(1):151-60. Epub 2007 Nov 8.)

adapted from CrossFit Endurance