Seems when you train hard you improve. Continue the process and improve more, but then, in time one may find they get tired, perform just as we'll, but require more effort to do it. After some more time running becomes more of a chore, performance might even decline. One notices recovery takes much longer and mentality becomes lax.
So the bottleneck? We look at many variables such as mileage, intensity, lactate thresholds and other things but does one talks about the endocrine system? Breaks do work, but during the ramp up, what is it that we can index that can keep us from the plods? Is there some way to objectivity determine when our endocrine system isn't keeping pace? Is subjectivity effective?
Thus assuming it is endocrine related to our training potential might suggest a way of viewing training from a different perspective. I would love to read what others have to write about this.