Quote from Fred Matheny (a well respected cycling coach): " No Man's Land" (NML) workouts provide a kinaesthetic sense of working hard but expose the rider to too much stress per unit gain. Instead most base training should be guilt-producingly easy, and the top-end, high intensity training (HIT) should be very mentally hard, not sort of hard".
I have spoken to a few elite fell-runners who follow exactly this programme. It's really hard to keep the slow pace though!