Truthfully, the best way to gain STRONG cardio is by doing this. You have to do this every other day, after 2-3 weeks you'll notice a difference, no question. On a treadmill, Walk for 1.5 miles at a speed of 3.0 mph, at 1.5 miles increase speed to roughly 5 mph, at 2 miles, increase to 7 mph. at 2.5 miles, decrease to 4 mph. Finish at 3 miles.
This build long term stamina. Extremely effective