I see a couple things to take away from this:
1. Always assume your attacker is armed, period. Whether knife, club, firearm, or even simply empty hands. With training, empty hands/feet can be every bit as deadly as using a weapon with them.
2. Time to react is crucial. This means you have to MAKE additional reaction time. Being aware of your surroundings, alert to suspicious behaviors, and use mobility/environment to your advantage to interfere with a potential attack.
Many of us already keep to these things.
While it's impossible to be 100% alert 100% of the time, certain things can be done to habitually place you in a better defensive posture. Where you sit in a restaurant, how you sit (to cover any defensive moves), who is with you, when and where you travel, learning not to ignore that "gut feeling", learning how to recognize targeting/stalking behavior, learning how to maneuver in the early stages of potential attacks to increase your defensive posture, etc.
I think, in addition to being alert, it's as important, if not more so, to actually recognize targeting/stalking behaviors for what they are. You often don't need to see a weapon or be in the process of being attacked to see what's coming down the pike many times.