As a former Marine, I can assure all readers that the OP is fiction. How do I know? Because psychotic ex-Marines fresh back from [name of warzone here] do not randomly hit college teachers. If said psychotic ex-Marine is going to do anything at all, he’s gonna shoot you with a gun. It’s a Marine Corps tradition. For further info on said tradition, please see articles on Charles Whitman and Lee Harvey Oswald.
Mate, the OP is fiction because they don’t let 15-year-olds into the Marines.
I come from a country with compulsory military service for men and they teach you get a black belt by the time your compulsive tour of duty is over and all things being equal, the guy with the training does better but it is pretty tough to overcome large physical advantages. A large strong guy with 4 years of current training is probably not going to be subdued by an out of shape 62 year old with 30 years of training.
I will say that training CAN help you fight off even multiple attackers of almost any size if they are not well trained.
E.g. a 120 pound woman with martial arts training might be able to subdue a 200 pound guy who has no idea what he is doing or even several 200 pound guys; but she has little chance against one physically fit 200 pound guy who has even rudimentary fighting skills unless she is verwell trained. At some point, your training gets good enough to be lethal and then you can handle yourself no matter how big the other guy is but it still requires a significant skill level advantage.
This. Mr. Miyagi exists only in fiction.
I had martial arts training as a kid. (including how to avoid situations by being aware of the environment.)
I’ve actually found this useful in “daily life” kinds of things. I’ve never actually needed to fight off a mugger, and never had a “violent idiot in a bar” situation… though I can’t exactly tell you why.
I think because my training subtly included “keep you ass out of trouble” training. If people are or have been drinking too much, I simply don’t hang around. (and I’m strange perhaps in that I simply don’t drink, because I HATE the effect alcohol has on my already quirky nervous system.)
Daily use, I have an chronic illness that occasionally causes rather spectacular falls. (just all of a sudden plotz, knowing how to BREAK FALL, may not have saved my life, but certainly saved me seriously injury… including a fall in my late teens /early 20s when I fell almost all the way down a flight of stairs. Just this alone has been a VERY useful skill for me.
My illness recently caused some falls where I wasn’t even aware I was falling till I hit the floor, I had NO chance to break fall… so this is something I need to bring up when I see the doctor next week. (and yes, huge bruises, and a fractured wrist, oh Yay!) These are the first times I’ve fallen “like a normal person” since I was a preteen! (It may be time to start relying on a cane… )
If I’ve been out at night, and have to walk to my car… I don’t walk close to the buildings, but rather out near the edge of the sidewalk. I go directly from point A to point B and I walk FAST. (don’t look like an easy target) I’m attentive and listening to what’s going around me in the environment.
As a PARENT, Martial Arts training was VERY helpful… Oh those trained reflexes were perfect for grabbing a toddler’s collar as she dashed toward the street… or caught a cup of juice as it fell off the table toward the floor.
Perhaps, the best encounter, the fight you REALLY win, (IMHO) is the fight that never happens. The only war you win, is the one where no sword is drawn.
Hey! I KNOW chefs! they can be SCARY tough… (lots of competition in chef training/hiring etc) and they DO have big big knives.
Your training probably wasn’t all that subtle about it. 
We teach that concept specifically as a self-defense technique: control your environment. Don’t put yourself into potentially dangerous situations if you can avoid it.
And I’m glad to see that you’ve read Sun Tzu: