Perplexed by my inability to "go fast" in this workout, I emailed the Mystery Coach in the hope he might be able to figure out what went wrong and how to "fix it." MC kindly provided a detailed response, including this chart. The chart has times across the top indicating the length of time the heart rate and paces below can be held. There's a range of paces that bracket my current fitness on line 9. From the first column, I should be able to run 6:11 pace for 12:33 with an associated heart rate of 197.
MC believes my inability to get my legs to go fast comes from the inability of my upper level muscle fibers to generate the force needed to drive you HR up because they have not been activated (lack of leg speed work) or because they were not conditioned properly in the base phase. He suspects my running ahead of my maximum lactate steady state (MLSS on the chart) has undermined my condition and recovery, exhibiting a disconnect between HR and running pace.
MC suggest I do some 15-60 sec accelerations with a lot of rest -- 1-2 minutes worth every 15 minutes. On the conditioning end, start with a 20 minute steady run at about 7:40 pace (HR should be about 163) with a 15-25' and cool down. This could be the day before a longer slower run (75-100 minutes).
For at least a few weeks, I should stay out of the red and orange zones (except for the accelerations).
His advice is pretty similar to my intuition. Although I've been enjoying the absurdly low heart rate (my run today was 6.2 at 9:19 per mile and 124(!) bpm average), I can't quite believe the low heart rate reflects my real fitness. I think I need to cut down on the LT pace until I get some semblance of normalcy. What he suggests is actually pretty similar in spirit to a couple of the Joe Rubio workouts (continuous "80%" runs and 200m intervals) I was doing early in the summer.
I do have to disagree with line 9 on the chart though. I really think I'm in better shape than that, which makes me ambivalent about changing what I'm doing. I think all this HM and 10k pace got me in better shape, but I recognize there are diminishing returns there.