I'll say to the people who don't believe in threshold, the cynics and the skeptics, I'm sorry for you. I’m sorry that you can't dream big. I'm sorry you don't believe in miracles.
Joking aside, I think we know we’re ultimately an experiment of one. For me, I started on the “classic” 1 long run, 1 tempo run, 1 interval, and 3-4 easy runs template. This worked of course as I was starting out, but I plateaued. I tried adding more volume but always found the interval session seemed to beat me up and zap my stamina.
I have tried higher mileage and two interval sessions a week, lower mileage and two interval sessions a week, higher and lower mileage with just one interval session a week… all permutations lead to, what I felt were, below optimal results. This, along with curiosity and a desire to optimize my time spent training, lead me to adopt a 2-3 tempo sessions per week approach.
I have really detailed data on all my training and checked this so that I’m not just idealizing what I did. The first time I used this approach, in 2017, for a 24 week period, I averaged 10.5 hours per week of running, with a high of 12 hours and a low of 7.5 hours. As a side note, this was and is my highest ever, I since have stopped running doubles and aim for 9-10 hours in singles for 7 days or 8-9 hours if running 6 days per week. During that time I ran 2-3 tempo sessions per week (if I did 3, I included it in the long run for that week). The sessions were very basic and consistent, 12*1km w/ 200m, 6*2km w/ 400m, 4*3km w/ 600m, 3*4km w/ 800m.
My HR_Max is 195 (tested, not guessed) so it’s pretty easy to know where I am as a 5% change is almost exactly 10 bpm. On those workouts, my HR never exceeded 90% HR_Max and basically stayed in the range of 80%-85% HR_Max. Part of this, I believe, was due to the volume I was running, as well as the fact these were in interval form. I find that if I can get to running 75-90 minutes average per day run, this gets me to the same fitness as if I average more, say 90-105 minutes, which I was doing during that time for some weeks. On 60 minutes average per day run, I know I have a little more fitness to squeeze out of myself, but I’m not far off from my best on that volume, particularly at 5km.
For some performance specifics, I ran a 10km time trial as fast as I ran a 10km race a year earlier and did it at 88% of HR_Max! When I ran the 10km race I ran a PR and did it at 94% of HR_Max (about 10 days after running the time trial). I also have other data, from testing, that shows even when I maintain volume if I start doing VO2 Max intervals, after just a few weeks, my performance declines. Also, if I drop the volume of my threshold sessions to 30 minutes, twice per week, at LT (instead of a little slower) my fitness is not as high as when I did 48 minutes at slightly slower than LT.
I know most aren’t likely interested in this level of minutia but as others have said, trying to get the most out of our training given time constraints and other obligations is the goal for us amateurs. Knowing we are doing that, at least for me, is a satisfying feeling and makes the training all the more fulfilling. Plus as others have said, the sustainability of it is most satisfying and impressive. Yes, you’re tired, but never trashed if you keep disciplined pacing.