I've dabbled in 6 different edged weapon martial arts, have a large DVD collection of the arts that I've studied and review tape all of the time. How much is it on my mind? All the time. Morning, Noon, Night and even in my Dreams
That is the thing with using knives. You have to practice, and use them all of the time, for you to be able to deploy them under pressure. I am not saying that you cannot do it, I've done it, but it is not the same thing as taking up knitting, you must live it. It is going to be messy.
OK, now what is the largest folder or fixed blade that you can legally carry? Carry that knife. You can use a 1/2 inch knife blade, but I'm willing to bet the law allows for 2.5 inch to 3 inch, so use that, IF that is what the law allows.
Under pressure, Fixed is better than a Folder, but folders do work too.
To start off, I suggest you look at videos on YouTube of knife fights, self defense, etc. You should get training, but in a pinch, practicing what you see in a video is close enough and better than nothing at all. When you have time, you should seek out martial arts training with a knife at an escrima, arnis or kalis school. Buy and collect DVDs of the methods of training. Then practice.
If you intend to thrust, you need a point on the knife.
There may be 12 or more different strikes with a knife that are possible. A horizontal slash and thrust, left-to-right, and right-to-left. Upward and downward strikes. And finally an "X" pattern, the diagonals. You need to defend and attack on these lines.
I suggest looking at the Spyderco Yojimbo, Yojumbo, Native, or Native Chief in the legal carry size.
I'm 60+ and I'm not running, so I plan on it getting very messy.
I support the 2nd Amendment Organizations of GOA, NRA, FPC, SAF, and "Knife Rights" and you should too!