agreed, this is primarily a question of the source/cause of fatigue.
the debate is central vs. peripheral fatigue, and as with most things, there are probably contributions from both.
some fatigue questions are relatively simple, i.e. why can you not sustain 100m pace for much longer than 200m? answer: depletion of readily available phosphocreatine stores and drop in blood pH due to rapid efflux of hydrogen ions from muscle due to large amounts of ATP hydrolysis.
in longer events the answer is not so clear.
i think in events such as the marathon, when one 'hits the wall' glycogen depletion is probably the main cause, plus a central fatigue (brain) contribution. specifically this is because you can't use arm glycogen to fuel your quads. your muscles lack an enzyme (glycogen-6-phosphatase) that would potentially allow glycogen to enter the blood as glucose. your liver has this enzyme, so liver glycogen plus exogenous (eaten) sugars are primarily responsible for maintaining blood glucose levels. there could potentially be a small contribution of gluconeogenesis (making new glucose from sources like certain amino acids) as well.
to get more specifically at OP's question, i would suggest that fatigue in the HM that does not allow you to run that pace for a marathon is from multiple sources:
1) central (brain) fatigue
2) peripheral fatigue (accumulation of certain metabolites, of which lactic acid is NOT NOT NOT one, and the lactate anion is a GOOD and useful energy substrate for the heart, liver, etc), unfortunately i'm not familiar enough with the area to suggest what these metabolites might be
3) possible exhaustion of blood bicarbonate/phosphate buffer leading to a decline in blood pH,
4) inability to bring the pain! for that long
5) if that pace was attempted for a full marathon, it would likely lead to glycogen depletion, and i think it's likely there is something with your 'inner pacer' realizing that this pace is not maintainable for double the duration, although i think sometimes in pursuit of a brilliant performance you may have to ignore that little voice to a degree, in order to find out your limits and all that
very interesting topic - sources of fatigue. there's gotta be somebody on this board who's studied it for a master's thesis or PhD dissertation...or maybe try sportsscientists.com, they usually have pretty legit stuff up