Sorry for the slow reply. Steve and I have been in Chamonix for our workshop.
By stacking longer aerobic workouts back to back you start the second workout in an even deeper glycogen depleted state which has a powerful stimulus on increasing the aerobic capacity and fat metabolism.
Doing a longer Z1 after an ME is smart for aerobic restoration after the hammering you gave your legs in the ME workout. This speeds recovery and maintains that aerobic base.