I would caution against a) focusing too much on HR or b) worrying about "getting as close to the line as possible." HR can vary based on many things unrelated to effort - for example, when it's below freezing I can run hard 800s and never get my HR above what is the low side of my heart rate range during a marathon. Plus, since your HR can lag in responding to your effort, it may not accurately reflect your effort during shorter duration intervals.
Darkwave: Thanks again for the thoughtful reply, though I may not take all of your advice. I have centered my training around HR for the past several years and it's going to be a little late for me to switch focus at this point! But don't worry about me too much; I think you may be slightly misunderstanding what I'm getting at.
Actually, the only workouts that I do WITHOUT a HR focus are specifically the I/R workouts (for the reason that you mentioned). So during the workouts I was not focused on HR at all. HR wasn't even on the watch face. I asked my original question before I had even looked at the HR data. The original question was about how I should be feeling; as an adult, I have always run alone. I've never had a coach. And because I don't do very much work in the I/R ranges, I'm not sure how good or bad people feel during that workout. While 5 bpm may be noise, if my goal at R is 97-100% HRmax, it means I didn't even reach the lower end of that line. But that was something I'm seeing after the workout, rather than during. I was at the time--and remain so--extremely satisfied with that workout. To me it was damn close to perfection... I was just trying to confirm that I was feeling perfection rather than "staying in my comfort zone" and maybe avoiding the work that I should be doing.
As for the general training methodology/ideology, I appreciate the caveats about using HR in general, though the fact that HR is affected by things other than effort is, to me, precisely the ADVANTAGE of using HR as a guide on longer runs. On 90% of my runs, the HRM comes into play only to rein me in. On about 10% of runs--the M, T, and progression runs--the HRM performs a service on both ends, keeping me back when frisky, but also reminding me to pick it up when the concentration begins to wander. An attentive coach might perform the same service.
In a nutshell, the "things unrelated to effort" such as temperature, terrain, recovery (full or inadequate), sleep (sufficient or inadequate), level of hydration, brutal hangover, headwind/tailwind... am I leaving anything out? We would all say to our fellow runners something like: "You may want to adjust your pace today due to the (choose one or more of the above). My position is that every one of those things registers in beats-per-minute on the HRM. (You can call this an unsubstantiated claim, because I don't have the time to search for scientific studies...). I would further contend that every one of those things IS, in fact, VERY related to effort (though maybe only related to "perception of effort," a whole 'nother can o' worms). So, just as you might tell a runner to "take it easy in this hot weather today and try not to blow up on the uphill stretches...try to keep the effort even rather than trying to dial in on a certain pace," my HRM tells me EXACTLY the same things (I claim).
This is not meant as a criticism of you, darkwave, or your advice, because I do really appreciate your generosity. I guess I tend to take every opportunity to evangelize about the benefits of understanding and using a HRM as a training tool whenever anyone cautions about using one. So I'm not discarding your advice at all; on the contrary, your posts (and The Stone Cutter's approbation of them) provide the kind of perspective through which a guy can re-evaluate his own beliefs. But in this case, I'm gonna stick with my ideas for the current cycle.
A two cup of coffee post. I need to get ready for work.