I am glad I found this thread, as it addresses a topic I am very concerned with.
Scott, to be honest, I do not fully believe that it is 100% correct to say that you shall never get out of your desired HR zone, regardless of the duration of the workout.
To me, it seems that the mechanisms of how a muscle is worked (which metabolic systems are used, how high is the intensity as a % of max. output, …) depend purely on the force acting on it. This, on the other hand, is represented by pace (or, for cyclists, by Watt output), much rather than by HR.
So, when I am on a treadmill (so pace is always constant), and my heart rate is within a certain HR zone at the beginning (first 30 minutes or so), then I think you can be quite sure that you are working the tissue the way you actually want it, even though the heart rate might drift later on. I think this is supported by the fact that ANY advanced road runner that I know is pacing purely by speed, and not by HR.
Following your advice, I would stop the workout when my heart rate drifted above a certain value (AeT, for example). And I think there is just no reason for that.
I think the reason why HR is so popular especially on UA is that, on natural terrain, the track is so diverse that you just can not “pace” by a simple measure. Pure speed obviously can’t do the job, as it neglects vertical, and vice-versa. To that, terrain surface (rock/sand, how high are the steps, …) adds even more complexity. So the most reliable thing for controlling intensity, for working out outdoors, is surely HR, but one should always keep in mind its weaknesses.
Myself, I train 90% of my volume on an incline trainer, where pace is 100% reproduceable and constant. In this case, I just do not see any reason to stop my workout if I drift out of my zone after 90 minutes or so.
I am really interested what you all think about all of this.