This is where a lot of people and I do mean a lot get confused. People view cardio as I get on the treadmill and do some boring ass slow jog for 45 min and boom I did cardio when in reality you just trained your aerobic system. The aerobic system in my mind is useless to train because its trained all day with walks in the mall or around work. Now what I believe is true cardiovascular endurance comes from the ATP/PCr or lactic threshold training. So lets explain as your body works it burns sugars as fuel the exhaust from this is lactic acid and once your body can't clear the lactic acid as fast as it needs to for your training you are forced to slow down or stop this action is your threshold. The way to increase this is by doing a work to rest ratio of work, 1 rest 1 or less. An example of this is kettle bell swings for 1 min with 45sec rest with a work total of five minutes. The goal is to not give your body time to recover and really push that threshold. Be warned you will probably puke doing this the first few times and should be done post workout