Dear NH,
Thanks for the second effort. I tried it myself and experimented with a negative split approach. I came in a slightly above the parameters suggested but felt more comfortable. Something to consider for the future.
Sometimes we must remind ourselves that we are training the physiology and must leave the intellect and psychology on the sideline for certain types of workouts. The 15x1:40/:20 is one such workout.
Keep up the good work.
cheers,
lg