I know some of the theory in fighting. For example, I know that it takes surprisingly little force (relatively speaking) to kill a man by crushing his larynx. I know which part of the fist should contact the body of your adversary (the major knuckles, that is where the fingers start, not the minor knuclkes in the middle of the fingers). I know that the most effective defense against a male aggressor who has a grapple on you is not, as is often believed, to kick or punch his genitals, but rather to squeeze them very hard (for some reason, a sudden attack in that region takes some time before the pain comes up fully, while a squeeze hurts instantly).
I know many such tidbits of information, but whether I'd be able to piece them all together in practice? No idea. Last time I got in something that could be called a fight was in middle school, some fifteen years ago. I have decent reflexes, but I'm not what you'd call muscular.
I might stand a chance against people of my same physical build, or perhaps slightly more muscular yet with worse reflexes, but if I ever find myself in the position of having to fight against anyone with any sort of training I might as well give them what they want right there and then and spare myself a beating.
For this reason I curse the country I live in every day for keeping tazers illegal, and I'm currently investigating the possibility of using a slightly unfocused, high power green laser as a personal defense weapon, the theory being that you momentarily blind your attacker by shining the laser in their face, then haul ass while they can't see you.