I know nothing about martial arts.
Would shadowboxing/kicking be a good cardio workout, as well as help in a fight?
I would do this alongside fullbody weight training.
I don't own a bag, and my gym doesn't have one.
How the fuck are you going to shadowbox when you don't train and don't know shit about boxing? Are you just gonna flail your arms and legs like a retard and pretend that you're in a Rocky movie?
So you train to how break your hands and fuck up your wrists (and possibly your thumbs, considering how retarded you are) on imaginary skulls?
Getting into bad habits is easy. Getting out of them is hard.
Nothing gives you more bad habits than doing random shit with absolutely no help.