What exactly is your 'aerobic threshold'. I hear that the Kenyans do the majority of their runs just below this (im guessing below means slower). What %HR max is this? Is it tempo pace (88-90%)? Or slower? I hear its where your lactic acid starts to build up but is it true that for a while this will build up but you can sort of survive in a sort of equilibrium between producing the acid and removing it? So is the Aerobic Threshold (AT???) the pace before ANY of this lactate is built up or at this equilibrium point? Should I incoportate this pace into most of my runs or is it too hard?