This takes more training then many martial arts places are willing to admit. I used to do Tang Soo Do, currently practice Tae Kwon Do. I enjoy it. I am slowly getting more fit. I genuinely look forward to training. And I am slowly shaking the rust off my technique even as I learn the TKD forms, which are different from the TSD forms I learned many years ago. And, I enjoy the traditional aspects of TKD, even as I realize it isn't nearly as ancient as TKD masters say it is.
I hesitate to say this, but I am starting to think traditional martial arts isn't all that great a way to learn how to fight. Put another way, TKD will not turn me or you, into a killing machine anytime soon. If I were looking mostly for self defense, I would not suggest traditional martial arts. They offer a path to self defense, but it takes a lot longer than people realize. (like 7 years or more) I don't say this to be critical, but back when I was in high school, I looked up to our black belts as these amazing fighters. I cannot say that is the case for every black belt I see now.
If you want to learn how to fight, go to a gym that does full contact fighting from the start. For striking, MMA, kickboxing and boxing. For grappling, try wrestling and Brazilian Jiu Jitsu.