I dont know what it is about the wind sprint type exercise but it really works. My body got used to the routine I was doing and results stopped. Nan experimented with me with this type of exercise (interval); although I argued with her, I tried it and began to get almost immediate results. Of course I thought I was going to die at first even though I was in (what I thought) was pretty good shape.
Now it has become a part of my routine and I just automatically do 20 to 30 second bursts of speed and then the same slower. I have worked up to thirty minutes and am well pleased with my endurance and overall results.