Intensity- how low is too low for aerobic development?

He is absolutely correct as always! Stay in your basement and learn from the best

Check the Ingebrigtsens' training.

Only a moron would write that. Be quiet, moron.

The OP is about a masters runner training 7 hours per week. I'm not convinced that a recreational runner can cherry pick bits from an elite runner. However, how an elite runner trains can never be irrelevant.

Are you a masters runner? Are you asking for your specific situation?

I see now.

I agree that this is a great topic, and relatively understudied.

I think everything is contextual, depending on what distance you're training for, what you do on your harder days, and your injury history. I'm a masters runner who can run about 97% as fast as I was at my peak at distances from 5k-10 miles, and near peak fitness for longer races. I always keep my average HR on my easy days below 70% of max (between 123 and 128 for me), and that has worked very well for me. I could probably get away with slightly faster, but I find that I never get injured (knock on wood) when I run that intensity on my easy days, and so I will keep doing that until I have a reason to do otherwise. Life permitting, I run at least an hour a day.

I do three workouts a week, usually two threshold sessions and something specific for my next race.

My first coach was the great Doug Bell, one of the best Colorado masters runners ever. I was also shocked by how easy he ran on his easy days. As a masters runner, he ran most of his easy days around 7:30-8:30 minute pace even when he was a sub-15 guy, and 8-9 minute pace when he was demolishing age-group records year after year in his 50s and early 60s.

Ed Whitlock was also famous for running very high mileage at very low intensity (2 minutes per mile slower than MP, if I remember correctly), and he might be the greatest masters runner ever.

The most extreme example of "easy days easy" in terms of elite runners I've seen is Yuki Kawauchi, who runs his easy days at 8-minute pace, which is more than 3' per mile slower than MP. And he seems to be doing all right. Might have something to do with how he's able to race as much as he does.

Point being, particularly for masters runners, there's ample evidence that you can achieve whatever level of success you might aspire to achieve while taking your easy days very easy.

Yes. I do about 1/3 quality. But I estimate it at more like 20% when including the cross training.

Very impressive/interesting!

What is your best 40+ 5k time and what kind of training did you do?

15:50 at 40 when I was running about 40MPW.

The definition and interpretation of easy/low-intensity training zones in the context of heart rate percentages can indeed vary, and different training methodologies may propose distinct ranges. The wide range of 62% to 82% of max heart rate reflects the diversity of opinions on what constitutes an easy or low-intensity effort.
JD's definition of 65% to 79% introduces a more specific range, suggesting that easy runs can be performed at the lower end. This is consistent with the idea that easy runs should be manageable and contribute to recovery. However, the concern about running VDOT easy paces at 65% of max heart rate raises an interesting point, as it prompts a consideration of the practicality and feasibility of maintaining such low intensity while still achieving aerobic benefits.

Jeff Galloway says walking is good aerobic development and he's an Olympian so good enough for me.

Galloway never walked when he was an olympian. He is just trying to sell books.

There is a massive amount of research around this topic, but runners struggle with believing in and following the science.

The most important endurance training takes place at or below LT1. Zone 1 in the three zone model. Zones 1 and 2 in the 5 zone model. At or below FatMax/Aerobic threshold/2 mm lactate/conversational pace/1 mm lactate above baseline/13 and under RPE. All training done in this zone improves all zones above it, namely LT2/Threshold and VO2 Max. it also builds capacity to do more training at higher intensities. it is the 80 in 80/20 training.

Most runners, even competitive runners are incorrectly overestimating the effort/pace at which this work should be done—to their detriment. LT1 is not a fixed heart rate effort and formulas can’t account for the fact that, if done right, it should gradually shift upward. If done wrong, runners are literally training themselves to be less metabolically efficient—-too much training above LT1 will make you worse in the long term. When done right, as with the best long distance runners in the world, humans can run at shockingly fast paces and remain “aerobic.”

All other aerobic sports have better application of this form or training than running: cycling, Nordic skiing, triathlon, swimming, speed skating….

Runners testing based on superstition and the scientifically false belief that there’s a minimum effort required for endurance training to be beneficial.

There is unfortunately not so much research if you look more closely to the Zone 1, in a 3 Zone model, as S. Seiler over-simplified it. He defines the boarder between his Zone1/Zone2 (3 zone model) as 2mmol. The problem is that there are two easy zones inside this Zone 1. One is the zone were there is no lactate rising above baseline, the other zone is in the area of 1.0 - 2.0mmol (individual of course).

This is reflected in the Norwegian zone model (e.g The Training Characteristics of World‑Class Distance Runners An Integration of Scientific Literature and Results‑Proven Practice). They have a 6-zone model, were zone 1 (60-72%HRmax), refelcts the zone were there is no raise of lactate above baseline, and zone 2 (73-82%HRmax) is the zone of around 1.0 -2.0mmol.

As we train around 80% of our time in the Norwegian Zone1/2 there comes the interesting question which time distribution is optimal inside this Zone1/2 and what is optimal for fast twitcher or slow twitcher. This is not answered clearly in science.

Figuring out I could run 8:00-9:00 pace on easy days instead of 7:00-7:30 and still be just as fit was a huge eye opener for me. Strava ruined easy pace and I’m sticking to that theory.

I believe Kellogg suggested 60-65% mhr would give you these benefits while making it easier to recover. Basis of 70-75?

Indeed. It might almost be better to ditch the gps watch on easy days, just run with an old school timex. Easy should feel easy. Lot of people let ego get in the way when they see pace.

Can second this. Every runner, every day ist different. It is important to cover all bases and there is definitely some point where it gets uneffective. But even for elite athletes some running at very slow speeds is ok. The pace should come natural and feel good on most days. No problem to run 7-8' per mile sometimes even when you're world class.