Realistically, what are the chances some guy a football field away is going to start a running charge with a knife toward you?
There's alot of "what if's" here. If the guy is a football length away, you can get to a safer place (i.e. car, home, office building, etc..) before he reaches you. Or you can stand there and take your time drawing your weapon which would be a breach of concealed carry fundementals (if there's another way out of the situation besiade drawing your weapon, do it).
If the guy is close to you, don't waste time. Draw and present your weapon. Presenting your weapon is not against the law. He is presenting deadly forces to you with his knife, you are defending yourself, and presenting deadly forces to him with your firearm. If that doesn't faze him, shoot to kill.
Of course, I live in Montana, and our gun laws are pretty good here. Damn near everyone if carrying a piece here...lol.
Some other states may have some weird engagement laws, so maybe I'm wrong depending on the state. If so, then my apologies.