Stephen426 is Right On.
I agree wholeheartedly with Stephen426. When your attacker has a knife, you are NOT gun-training, even if you have a gun. You're knife-training. Knife-training is about adapting empty-hand martial arts. A good side step, a good side kick, these are things that are used to defend against a knife. Guns can ensure a mutual slaying, but that's all. Guns are purely offensive weapons. (With the exception of large rifles, which point the stock can be used as a club...)
I have only two major points to add: First, you're going to get cut, and second, I'm not a TKD guy...
Point one: You're going to get cut.
I practice Iaido and Kendo, among other things (Isshin-Ruy Karate, Muso Shindo Jodo, Aikido...). So, I play with sharp things quite a bit. I will occasionally train with live swords, and for a long time, a live sword was all I had (it was cheaper than a dulled sword). I have cut myself many, many times training with blades. And, upon entering our class, my Sensei and I will stress one MAJOR point with training: If you train with blades, YOU WILL GET CUT.
How is this at all relevant to the present discussion? The point is, if you are training against a knife attacker, expect to take damage. The "21-foot test" is inherently flawed, because the test is to see if you can shoot the guy down before you take ANY damage. This is simply not going to happen in real life. If you are attacked with a knife, you WILL GET CUT. The trick is to minimize your damage taken. Stephen426 is right on, because he advocates pushing off of the attacker (with his kick). Get distance between you and them... But this brings us to...
Point two: I'm not a TKD guy...
I can swing a sword with the greatest of ease. I can also punch, arm lock, and throw with moderate competence. I can't kick worth a flip! I can barely raise my leg to knee-level, and I wouldn't try to kick the knees out of a running assailant in a time of crisis. So, am I defenseless in this situation? Of course not!
The point is not to "deliver a side kick" per se. The point is, if a knife assailant is running toward you, GET YOUR GUARD UP! Put your hand out, and try to absorb the damage in your extremities. Don't take the knife to the chest, take it to the arm or the hand! You'll live. You'll have to go to the hospital for stitches, but you'll live. You can live a long time with your arm completely severed. A penetrating stab wound to the arm is survivable... So is a stab wound to the leg, if you can raise your leg that high...
"But, Samurai," you say, "How can I draw my gun if my arm is cut up?" That's why I use the singular "hand" instead of "hands." Take the damage in your non-weapon hand. Keep your weapon hand at low ready, held in close to your body. Extend your non-weapon hand to keep the attacker at arms reach. Shoot, shoot, shoot!!!
Now, we need to apply what we've learned!
Let's do a new drill. Start with a buddy at 21 feet. Have him run at you with a rubber knife. With the weapon on your right side, spin to the left, extending your left hand to meet the attacker. At the same time, draw with your right hand and shoot your friend from the hip as many times as you can. (Snap caps, please!...)
How do we know if the drill worked? If you only take damage from the shoulder down your arm, it worked. You killed your buddy, and you took survivable damage. Now, when it works (and it will work), repeat this excercise 1,000 times. Repeat it again. Then after that, repeat it again!
Train, train, train!