Vault, what are you referring to as MHR? I always knew this to be Maximum Heart Rate. What I think you are referring to, and correctly but just with the wrong name, is the Anaerobic Threashold (AT). Basically the point at which your body switches from burning Carbohydrates/Fats to burning Proteins which are harder for the body to consume. Typically this is about 60% of the MHR, if my memory serves me correctly.
Here's an article I just looked up from Triathlete Magazine that may help designing a training program:
http://www.triathletemag.com/story.c...92&pageID=1732