Run for your life!
I then theorized that what signals your body to grow is forcing it outside of its comfort zone. For fast twitch fibers, you do that by doing more, faster. For slow twitch fibers, you do that by exercising longer. However, the smooth and cardiac muscles don't have fast and slow twitch fibers. Therefore, how you raise them above their comfort zone doesn't matter so much as the fact that you do so. You could, therefore, increase cardiovascular fitness with short, intense workouts.
Biologically speaking, physical fitness should be evident in how attractive we find people. If you compare athletes, who are all in the peak of health, the one's with the "best" bodies are the sprinters. We intuitively recognize that style of body as the most successful.
A problem I had with this is that sprinting can easily cause injuries, but so can the repetitive stress injuries of running, so it may be a wash.
The one major flaw I can see to this theory is that humans are distance runners biologically, not sprinters.
Anyway, being of that sort of mind, I decided to do a rough test on myself. With a background in weightlifting, I already had a tendency towards fast-twitch muscles, so sprinting would be an easier fit. Here's how it's gone so far:
I thought I was going to die. I ran until I was winded, and I felt like I had tuberculosis. My lungs were complaining all evening. They were still kind of sore the next morning. As a general rule, when you know you have an internal organ, something is wrong, and I was worried about this one.
Nothing much different, but the next morning, I was breathing noticeably deeper. I don't think it could work this fast, so I'm guessing I just started using spare capacity that I already had, not developing new capacity.
Inner thigh muscles very sore.
Calves hurt. Not shins, which is surprising, because that's what always hurt when I ran before.
I noticed that I wasn't pushing myself hard enough when I slowed from running on the balls of my feet to the heels. Have to make myself go up on the toes.
It is taking noticeably longer to wind myself. The little parking lot I have for a track is kind of short to maintain a good speed. Normally cardiovascular exercise takes two weeks to show itself. It may be that I started from such a low level that the benefits are coming thick and fast, but it hasn't been that long at all.