I think you have hit the nail squarely on the head. Your metabolism is heavily biased toward the anaerobic end. Based on those numbers I call AeT at 140 or close by. You might want to do our HR drift test to see if this is confirmed. What was your perceived exertion at 140. If it was pretty easy feeling then chances are good that this is AeT for you.
Another good way to tell if this is an aerobic pace for you is: can you do the same workout day after day after day with accumulating fatigue?
Be patient and you will see steady gains for months on end.