My own suspicion is that Maffetone is a bit too enthusiastic about it, plus it seems like a lot of his big success stories are from highly talented athletes whose biggest error in their training prior to Maffetone was that they were running everything too hard. So there was a huge area of cardiovascular fitness in those athletes which was underdeveloped and just ripe for picking.
My own thinking on this has come around to the program that John Parker presents in his book Heart Monitor Training for the Compleat Idiot. His program uses both easy running (less than 70% of the heart rate reserve) alternating with bouts of harder running, where the runner does work above 85% of the HRR. Well, roughly alternating, and he warns that if you're going to err to one side of doing too much hard or too much easy, go with too much easy and take breaks if needed. The idea seems pretty good, keep the easy stuff easy, and that allows you run the hard stuff hard, hard as in not only will your heart rate be up in the zone, but you'll be running near top speed for that heart rate. It maybe doesn't do a runner much good to be running nearly all out by heart rate on overly tired legs, as that won't really help to build the neuromuscular adaptations needed to promote a strong and efficient stride.
But I definitely suspect that both Parker and Maffetone are correct in their assertions that probably most runners tend to run their easy runs too hard and too fast.
I agree about the athlete examples. It seems like it's the over-trainers that benefit most. Surprise surprise. Telling a recreational jogger to monitor their heart rate makes little sense to me, but I know a lot of people here have had great success doing just that.
For me, there are basically three paces: anaerobic (sprints), lactate threshold (tempo), and aerobic (easy pace). Heart rate, as I understand it, is a proxy measure, of VO2 max, if I recall correctly, which itself highly variable according to individual genetics, athletic background, and current training. Given this variability, for me at least, I can't see the sense of running by monitor and a standard formula rather than by feel, unless you're willing to diligently do your MAF tests on a regular basis. I
know when the pace is easy. I can run for as long as my joints, tendons, and muscles can take it. Cardio conditioning isn't the limiting factor (for distance). Lactate threshold depends on conditioning. When I'm in good cardio condition I can maybe run 5-6 miles at lactate threshold. Right now I'm in crap cardio condition and would be lucky to knock out a mile at tempo pace. Sprints are sprints, I can go 100 meters full out multiple times when I'm in decent condition.
The only question is what the ratio of these three paces with respect to one other should be. Everyone says the majority of one's running should be easy pace. The rest is details. I would imagine the more miles one runs, the greater the percentage should be easy miles. If you only run three times a week for three miles, you could probably do every run at tempo pace no problem, after having built up to of course.
Oh, and another thing I've looked into a bit is the idea that anaerobic exercise will interfere with building a good aerobic base. A lot of people disagree with this and say the exact opposite, that running will interfere with strength training, but strength training has little detrimental effect on running. I've found this too me true. My only scheduling concession is to avoid heavy squats and deadlifts before a long run day. Then after the long run I like 48s of rest before I hit the ST again, in order to build back energy stores.
BTW, anyone want to by my Garmin F10? Barely used . . .