I would say that the answer is -perform very short segments of high intensity activity
From my own personal experience in cross country, you want to always perform short bursts of activity. You don't want to work above your target heart rate because that is just asking for trouble. You can always run if that's what you can maintain, but most people can't. Keeping one foot on the floor is a bad idea because you will not be able to go as fast that way.
