Why Doing More Work Increases Your Cardio

There is a popular martial arts quote that I’d like to tell you about. 
“Conditioning is the greatest hold.” – Karl Gotch
Gotch’s quote simply says that no matter what techniques you (or your opponent) know, if you have better cardio, and can push the fight/match longer and harder, then you’ll have the upper hand.
Now, you might not be an athlete, a fighter, or even anybody who competes in anything right now.  But that doesn't mean you still can't apply this to your own life.  
There are many elements to a good cardio workout (and in turn, having good cardio), but one of – if not THE – most important is Work Capacity.
Work Capacity is a pretty simple concept that many trainers try to give complex definitions or major pseudo-scientific discussions. Work Capacity is essentially how much work you can do, and how hard/fast you can do it.  
In other words, if you it normally takes you say 20 minutes to perform XYZ task, then having more work capacity means you can get it done in 15 minutes.  Or, coming from the other end, it can allow you to do a lot more work in that same 20 minutes than you normally would do.
This is important because you are simply increasing the capacity of your heart and lungs.  They can pump more blood and oxygen, meaning that you are not only fitter and more healthy, but the normal tasks that were "hard" before can be completed fairly easily.  
It's kind of like a car's gas tank.  When you have a 10-gallon tank, you can only go so far at normal driving speeds.  Driving super-fast might mean that you can go an even shorter distance.  But, if you take that 10-gallon tank out, and put in a 20-gallon tank, you've just doubled the distance that car can go.  It doesn't matter if you're talking about driving speeds or with the gas pedal on the floor - you're able to go a lot further.
Work capacity is the same exact principle - only it's your body's gas tank.  Double the size of that tank, and you're going a lot further (meaning you can do more work, and do it easier than ever before).
To increase work capacity - and in turn your cardio - you simply have to do cardio workouts that make you do more work, or doing the same amount of work harder.  This way, you condition yourself to being able to do more.  
Continuing on with our example, instead of taking one gas tank out and putting another one back in, doing hard cardio workouts more or less takes the gas tank you have now, and expands it - makes it bigger.
Start doing cardio workouts that increase your work capacity, and you'll be getting in better shape in no time.