With Hadd training, this is a partially valid concern. One effect of the training is that, while you can improve and maintain threshold-level speed, your ability to kick or surge is limited. Your cruising speed in the short run will improve, but it’s hard to speed up from there.
Osler style base training avoids this by having you do a time trial or race once a week every week. This one hard short-medium run serves as your speedwork. Eventually, once you switch to sharpening training, you work on repeats and other speed frequently ahead of a goal race. But the weekly time trial helps work on and maintain your fast running during the lengthy base training cycle. (It also allows you to quickly take up speedwork, as you’ve been practicing it some each week)
One of the drawbacks of common marathon training plans that include speedwork is that the speedwork not only limits your base aerobic development, but your body can actually burn out on the repeated hard stimulus after about 3 months, very counterproductive if you’re planning to run a marathon in 3-5 months. (And no, a solid taper alone is not enough of a break to refresh those reserves)
Both Osler and Hadd agree that an ideal base training focused phase should last around a year or more before you begin any serious speedwork, and that most runners simply don’t do this because of their own lack of patience, not to mention short term ambitions of goal races. Most simply cannot fathom thinking, planning or executing that far ahead.
I realize one of the reasons my performance improved so dramatically in 2017-2018 was that I made a point to run easy almost all of the time. Most times it was out of necessity, running home from work and needing to guarantee completion of the run and arrival. I ran as easy as I needed to. Often, admittedly, fatigue from the workweek and rest of my life compelled me not to push it on most runs. I covered mileage and got home. It not only allowed me to pile a lot of weekly mileage, but the sub-aerobic-threshold running gradually pushed my aerobic threshold upward, as Hadd’s explained at length.
Most runners don’t do this. Most coaches wouldn’t recommend this. But my obsessive unrelenting focus on easy aerobic base running led to my lactate/aerobic thresholds consistently improving, which showed in my race results. I occasionally did some fast running, but I didn’t need to do much at all to see improvement during this period, even when the Racing Team was out of season and I was only running easy on my own.
A few training plans do lean exclusively on easy running, at least for the first couple months. IronFit doesn’t program any fast running until 7-8 weeks in. Hal Higdon’s easy plans are all easy running, and even his intermediate plans only include a slightly-harder-than-easy marathon pace run once a week. (I’ve mentioned before that while Hansons programs only easy runs for the first 3 weeks or so, I don’t really count that in such examples as that pre-speed base period is relatively short.)
But most plans ask for speed or tempo work right off the bat, holding that pattern through the entire 16-20 weeks. Perhaps that’s too much? Not because a runner can’t handle it: Obviously, most can. But it may be too much because, as Mark Sisson and Brad Kearns or Tom Osler have pointed out, such running is best employed in brief 1-2 month periods right before key races, when its benefits are needed. (In that respect, IronFit somewhat gets it right, only programming speed/tempo in the last 9 weeks of training… though even that’s a little long.)
One of the reasons I’ve found Osler training a good fit is because I think he found the right combination back in 1967 when he wrote The Conditioning of Distance Runners. Run easy, except for one hard run/race a week. Build that easy mileage for 3 months. Then pile on speed workouts in the 7 weeks before your big race or racing block.
In my experience, more speed/tempo each week than that can be too much after a few weeks. This may be why so many serious runners get injured or flat-line their performances so much.
In my experience, less speed/tempo each week than that can indeed stagnate your faster running and make it hard to kick. My long stretches of easy training did improve my cruising speed. But the best racing results only came when I was also doing speed and tempo work in training.
I realize Hadd avoids speed training entirely. However, Hadd’s Phase One base training places top priority on troubleshooting and building your aerobic base fitness only months, only gradually adding harder running once you’ve reached a suitable run volume and shown aerobic threshold base improvement. His training aims to troubleshoot your aerobic shortcomings first and foremost. You only worry about running faster once that’s been suitably addressed.
And it goes back to what both have pointed out about most coaches’ and runners’ general lack of patience, of wanting to see results now. We go to the speedwork right away because we want to see ourselves running fast. And we often default to the (somewhat faulty) common sense that you can only get faster if you practice running faster.