The eye must see all sides. The ear must listen in all directions. - from the Kenpo Gokui
The Zen monk, Thich Nhat Hanh, advises to practice mindful awareness with every waking moment. When opening the door, be aware of opening the door. Pick up the candle with two hands instead of one to accentuate mindfulness, and so on. That doesn't sound easy, does it? Think about really placing your full attention on every mundane task you do throughout the course of the day. To be thoroughly aware of every nuance and event within one's purview is the basis for meditation and the path to satori (enlightenment).
In classical budo the emphasis was on constant surveillance of one's surroundings. This made it to the modern ways as expressed in karatedo. Shotokan founder Gichin Funakoshi would demonstrate on his students how improperly held chop sticks could be rammed down the throat if one was caught off-guard. He would walk wide around turns in the street so as not to be surprised by a would-be attacker. In the dojo Funakoshi would deliver sound kicks to the back of the legs of students that he deemed weren't paying attention.
The problem with being on-guard all the time can be debilitating. In this way you're not alert at all, in fact, you're merely thinking about being alert. All one can really do is be awake, open-minded, and relaxed. Then the potential attack can be fended off effectively and swiftly without the mind being laden with a cacophony of "what if" scenarios.
The fact that our daily lives are invaded by ceaseless thoughts and preoccupations or "monkey mind" as they say in Zen are by-products of modernity and our culture. I consider the ability to ponder one's daily routines alongside of a job and whatever you have to do in the course of the day as multi-tasking, not some form of lower consciousness. Most people these days aren't cut out to be a Zen monk or a Japanese bushi.
Here's a good example of spontaneous right action to an attack, Spaghetti Western style. In the 1971 movie Red Sun, Charles Bronson's character attempts to ambush a samurai. We'll call this one a draw.