21 feet equals 1.5 seconds in start to finish for a human being to get from point a to point b, on the average. 1.5 seconds is also the acceptable time for drawing a firearm and getting off one or two shots for self defense. Whether you can do it faster or it takes you more time, the concept is more important than the mechanics. At the end of the day, self defense is about being able to evade the danger, not necessarily winning the fight.
Not getting into a fight at all in the first place is the biggest win in self defense. Combat is what happens when the fight can not be avoided, or one wishes to take the offensive. These are two completely different concepts which are often confused. Self defense employs combat as the last resort. Combat employs self defense as the last resort. This video is very informative, and gives great understanding in the difference between on and the other.