I have for many years realised the benefits of high intensity (hit) exercise and it does work... but like anything it has it's downside... illness and injury can become very common if diet and rest aren't factored in. I find the problem is that most people exercise in what I call the grey zone - not easy and long enough... or not hard and short enough. Most recreational runners fall into this category.... if you can balance easy and long (<140bpm) workouts with short and hard (20 minutes) you will gain many benefits. But you can't build an aerobic base without long easy workouts imho.