They say it's better to work smart than to work hard. But when it comes to your cardio workouts, that might not be the case.
Now, that does not mean you should go out and do anything that's dangerous, or that you should throw caution to the wind when performing your workouts. That's not it.
However, it is erroneous to think that just because a certain machine or type of exercise is "easier" than another, and that you can even "do more" (supposedly), that you'll get more benefit.
For instance, most people can run a lot further on a treadmill or elliptical trainer than they can outside in the same exact amount of time – and it's usually not as hard, either. But does that mean that more efficient, "easier" work did them more good?
Ok, we know that to improve your cardio, there's not going to be getting around doing hard work. Whether it's Long Slow Distance (ie – jogging) and the "hard" part comes from how long it takes, or it's High Intensity Training Interval (ie – sprinting), and the "hard" part comes from just how intense the work has to be, you're going to be putting your effort in.
And do not think that there are any shortcuts, either.
To increase your cardio, there are no technical improvements that will increase your aerobic or anaerobic capacities. You just have to do the work. And that means effort. And effort is hard.
While there may be certain exercises or methods of exercise that are more beneficial than others, the main thing that will give credence to your cardio workouts will be how much effort you're putting in.
For example, walking and running are pretty much the same thing – one is just a lot more intense version of the other. Same would go for doing a circuit of dumbbell complexes. You could do the exact same circuit with a pair of 50-pound dumbbells, or a pair of 2-pound dumbbells. Which one do you think will be harder?
You can not really say that complex training is "more effective" than running, or vice versa, because it depends on how much effort you're putting in. The complexes with 50-pound dumbbells will be much harder than walking. However, running at a good clip very well might be harder than doing complexes with 2-pound dumbbells.
See where this is going?
If you want to increase your cardio, you're gonna have to work – and work hard.
Train Hard, Rest Hard, Play Hard.