If you study martial arts seriously, you eventually internalize the moves and you don't *have* to think about them. They just happen. But it requires a serious effort -- you're not going to get to this point from a few gym classes.
When I was in high school I studied karate and jiu-jutsu very seriously for about 4 years, with a few years of casual training before that. One night I was completely drunk, barely able to stand, and someone punched me in the face with no warning -- apparently I said something to piss him off, but in my stupor I was completely unaware of it until the punch landed.
We both fell over. By the time we hit the ground, I had him in a jiu-jitsu "guard", with my legs controlling his body and his arms tied up so he couldn't hit me. I was drunk and surprised, and I certainly didn't have the time or wits to think about anything -- but I didn't need to think. I'd practiced the moves thousands of times and they happened pretty much automatically. (Some friends of mine were nearby and pulled us apart, so that was pretty much the whole fight).
Of course I trained almost every day for several years. And that doesn't make me unbeatable or mean that I have a good chance against a knife or gun. But it is possible to train to fight in a way that really makes a difference in a real situation.