I do my "Tempo" 2x a week on days in between my high intensity workkouts and they are done in spikes or flats and I stay on the balls of my feet. I find it difficult to hit my times when my heel hits when I am trying to run 75%. For me this is 28-29 pace for a 200. I do 6×200 with 2' Rest. One may argue it is too fast to recover from but I disagree. I feel I get more out of this type of workout and recover better rather than plodding along in sneakers at 34 second pace for 2200 meters.