Fast way to improve my Cardio?

run? If you have a track nearby, do 1 lap (400yds) sprints. You can also do 2 lap sprints. A decent time would be sub 1:30 for 1 lap and sub 3:00 for two. A great time would be sub 1 minute per lap. Also you could jump rope for 3 minutes straight although your legs might tire before you run out of breath. Or you could just ride your bike....