Tidbit I've always found interesting, based on some classes I took, I learned a man can close 16-20' quicker than you can draw and aim. Think about that....
One of my best friends is a handgun instructor, and I have the luck of being his "star pupil" at most of his classes. Another words, I get to be the one that runs the drill first to give the noobs in the class a chance to see what he is asking them to do.
After class one afternoon, we were doing the "man with a knife drill". I would stand facing the target in a surrender pose (hands up like a football goal post) and he would stand behind me with one hand on my shoulder facing away. After I gave the ready command he would leave, at a random interval, in a full sprint away from my static position. When I felt the hand leave my should I was to clear leather and get a shot off onto the target. When he heard the shot ring out, he would drop a weighted object like a blue gun or magazine as a place marker. When I was clear and reholstered, I would turn around and we would measure the distance covered. That was the minimum distance you would want a knife wielding attacker to get. It was spooky how far it was. I think my first time it was about 18', of course this is knowing what's getting ready to happen.
So, then we got the great idea of making a contest out of it....gotta love competition.

Making a sport of it, we were able to get down to 14' for me and 13' for him. This was from an outside of the waistband holster and no garments concealing the pistol.
Add in concealment clothing and the reaction time of realizing the crazy guy with the knife is charging at you, and you might need upwards of 30'. If you have a hard time visualizing 20', most full size half ton trucks are right at 20' long.
This is also the reason why you don't want to freeze when the clock starts. You have to explode off the X, and put some distance between you and the bad guy. Standing still like an old west shootout or like at a public target range will get you hurt (or worse).
When the bad guy is at bad breath distances, it's best to have some hand to hand skills. Also, a fixed blade knife is very quick to present.