How can you increase your cardio-repiratory endurance?

Aerobic exercises is just a way of saying that you aren't working harder than your heart and lungs are able to keep up, so you're working out at a pace that you can keep up for some time. Basically, the 100 m dash isn't aerobic, the demands are so great that you're basically running on what's stored in your body. A marathon OTOH is too long to be run on stored oxygen, so you can only run on the rate that your luungs can deliver - so it's aerobic.