I hurt my back when I was 42. Before that, my practice was always to go a little farther and/or faster than I thought I wanted to, and that served me well. After I hurt my back (L4/L5 disc) that approach invariably ended with me in pain and off my bike for too many days. I then started wrapping up every ride a little earlier and a little slower than I wanted to. I built up miles and speed slowly, but I experienced no pain, and I did build up miles and speed. HIIT is a useful technique, but probably not for everyone, unless 'HI' is redefined for every individual.