Numerous people have asked whether they should do some sort of cardio to make their Martial Arts better. My answer has always been yes. To perform at the best of your ability in martial arts you need to be in the best physical shape that "you" can be. And the key there is you.
Each person is going to have different levels of physical capability. Most peoples abilities decline as they age. So simply you should be in the best physical ability that you can be for you capability. Remember, the better shape you are in the easier your Martial Arts training will probably come to you. Some people will have physical restrictions that will naturally hold them back. They can still be at the best that the physical restriction allows.
The next question I get or that I see people doing is whether they should combine cardio and martial arts training together at the same time. NOOOO!!!!! Remember that Martial Arts is a skill. To further enhance that skill you need to practice your Martial Arts fresh. How can you practice these skills while being exhausted? Train fresh. If you are tired for example you will start to drop your hands while training. Your guard will be down, your stance will be off, and you do not want this to become muscle memory or habit. You will only become more physically fit but your skill will suffer tremendously.
My recommendation is the following. Do a short warm up. Foot work is great way to warm up and train your skill also. You could also do a little shadow boxing to warm up and get the blood flowing. Once your body is warmed up, start a 5 minute stretch to loosen up the body. Stretch your legs, arms, torso, etc. Remember being limber helps in speed, and also helps prevent injuries. So now do your training. Practice your drills while enhancing your skills. If you start to become out of breath, or getting sloppy in your skill, take a break. If you are at the point that you can not come back to the skill without being physically exhausted, then quit training your Martial Arts at this point. This is the point where you want to start to train yourself physically. Cardio or strength. Maybe if you are too tired you will want to skip any strength training in case of injury. Here is the point where you could do a run/jog, jump rope, burpees, ladder drills, etc. Anything that taxes your cardio. Remember.... Do not do punching or kicking here. Strictly non martial arts cardio.
How you might want to shape your day of training::: You might want to separate out the two different types of activities. If you can you should practice your skill earlier in the day and then your physical training. You don't want to lift weights for example and be sore and tired later when you train your Martial Arts.
AHHHH.. Can't I combine my training?? Like hitting the heavy bag for rounds. I see people do rounds on a heavy bag, or double end bag etc. I typically advise against it to my students. For most part I notice that everyone will get really sloppy with their rounds on the bag. And they start to engrain this into their skill. People train 2-3 minute rounds on the bag and they start to telegraph their punches, their hands are dropping. They are not changing levels. This is bad training in my opinion. If you want to train on the heavy bag, treat it like a skill training. Jab your way in.... angle, work on your head movement, work on your foot work relating to the bag, move with the bag, also work on where your feet are in relation to the bag when you want to strike the bag. Are you Jammed, are you square, are you outside range. When you start to get tired stop.
OK... Can you be totally honest with yourself and skill? Then sure... Train on the heavy bag. You will see boxers train rounds. Why can't I? You will notice the boxer will typically never train to exhaustion. They may get to the point where they are taxing their cardio/body. They will push a little to the point where they start to fall apart. Then it's time to stop. Probably a good time to go hit a speed bag. It makes you keep your hands up while still getting rounds in of cardio.
So that's always been my thought on physical training vs. martial arts. I have always designed my class that if we do any cardio it's at the tail end. And it varies. It's usually doesn't have much to do with any skill. It's designed to push the individuals cardio wise so that when they are in class the next time that during skills training they don't become physically tired as quick.