First off, it is best to remember that you can train for decades, be situationally aware, have a weapon, and still lose. The purpose of training is to shift the odds, not eliminate risk.
Bill M. asked in the comments for my take on defending in the situation shown in this video. My answer is that you can't. The instructor set up the student to make a point. By the time he started attacking, there was no longer an effective defense. Part of this was that it was an instructor/student interaction, but a real defense would have needed to start sooner.
A defense would start by maintaining distance. It doesn't have to be a lot, but it needs to be enough that an aggressor needs to take a full step. Then, don't get backed into a corner or up against a wall, keep a line of retreat, and continue to retreat, or circle. Don't engage, disengage. If he raises his voice, lower yours. Be submissive, let him think he's "winning". You do not want to fight empty hand against knife.
If you are carrying a firearm, remember that inside of 15 feet you will not get a weapon out of a holster before you get stabbed by a determined attacker, so whatever defense you intend to use, you will face this attacker with your hands. The aggressor in this video is Dan Inosanto, so the outcome is foregone.
There is no substitute for training. Regular, two to three sessions a week, every week for years, training. I do not think that anyone can learn enough to be effective in
any martial art in six weeks, or six months. A basic level of
functionality, which is about where I am, takes about a decade. You can't
learn this from a book or a video. Just like training with a gun, there is no substitute for a good instructor and lots of practice.
And to close this, I have no illusions about my skill level. I never want to be in a knife fight.