After doing my second amateur fight and losing my dad who was working my corner pointed out that although I am good, and i could have beaten my opponent, my cardio killed me. I'm not going to lie, I don't really do any extra cardio out side of TKD class. We sometimes jog around inside the school, but it's not enough to really show any significant progress. We also do jump rope but still it's not enough. I want to build my cardio as fast as I can, while still being safe. So I come here to ask, how to quickly build cardio level if at all possible to build it quickly?