What I do is I get my ass on the treadmill. Now just running will help but not as much as what I am going to tell you to do. What you need to do is warm up first (a couples of minutes should do it) then you need to find what is a good jogging pace for you. You should keep at this pace for around 2 minutes then you need to kick it up to a fast running pace for around 30 seconds to a minute (whatever you can handle). Then drop back down to the jogging pace for another 2 minutes then back up to your running pace and so and so on.
Once you build your cardio up some what I like to do then is after I am done with my running pace (which should be up to a minute by then) I like to add a sprinting pace. I will run flat out for around 15 to 30 seconds before I drop back down to the jogging pace.
Like I said before just running will help but it is constantly changing levels that will build you cardio up. If you do this routine I guarantee it will greatly improve your cardio.
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