Best cardio is to train hard. Other forms of cardio are Crossfit and p90x

Running at 5 in the morning pushups swimming . Shadow Boxing.

Kettlebell training within Tabata protocol will surely give you a good cardio and endurance workout. Practice plenty of hand to hand swings, snatches, power swings and all the ballistic routines with tabata interval training.. Can’t beat it!

As far as leg kicks, you should focus more on leg strength, speed, accuracy and power. Naturally, you want to keep your overall stretching routines going on a regular basis. But stretching is not going to improve your kicks unless you plan on doing high kicks. Also, be sure to do dynamic stretches prior to training. Static stretches before training will hurt you in the long run..

More important than stretching is to improve overall joint mobility. One can be very pliable and yet not have full range of motion.. There is a difference..

Good luck with your training..

Best leg stretches I learned in Tae Kwon Do (I know, it’s one of the few credible and valuable things I learned in TKD).

Splits and leg raises. Put your calf/ankle on a partner’s shoulder, and have them slowly stand up, raising your leg higher and higher. When you do splits, sit on the ground, and spread your legs out. Then have a partner grab your hands and pull you forward, while putting their feet on the insides of your legs, pushing them out and pulling you down/forward.
You do those stretches for a while and you’ll be throwing head kicks with ease.

As for cardio, swimming is great but I think some other people have better ideas for that. You could always run (sprints, uphill runs, stairs, etc.) Do some bag work. And also, just spar. Spar, spar, spar a lot.

Anything that gets your heart rate up while practicing the technique is good. Train like you fight.

Weighted swimming is good, as is parachute running.