I have been one to believe that a non violent man is a great man indeed. It takes a bit of wisdom to see that violence is not the only route and is also in fact the easy route. It take a great deal of patience and tolerance to do the non violence route and so is harder. I on the other hand like to fight and so am not a great man but I do know how to deescalate a fight especially if I am outmatched or outnumbered. In the case of the guy with the knife if you did not try to calm him down and instead tried to take him on you would be dead. The odds of an unarmed man winning a fight against a man with a knife, experienced or not, are not good. You did the right thing and I honor that. As for training in a MA it would be beneficial on more levels than making you a badass on the playground. You will get into better shape and you will learn that while you are being taught to beat the hell out of someone the teacher will almost always tell you to go the route you already took by avoiding any conflicts. For somebody of your body type I would suggest boxing as you will not have to do any high kicks
and also perhaps some juijutsu. Joint locks are always a must if you ask me and are good for controlling a situation.