Above mine too, so I pulled out Dr Phil Skiba latest book. There isn’t a lot of studies on low intensity cycling, because of logistics of finding athletes and how long the study would need to run.
But there are some studies, and some strong empirical evidence that spending time at low intensity is correlated with improved fatigue resistance, lactate clearance, and performance.
Think about it this way, you do sprint intervals to target specific muscle fibers and/or groups of muscle fibers. The same with vo2max intervals. The same with threshold intervals. Etc., etc.
Why wouldn’t you want to do the same with slow twitch fibers? A couple obvious reasons are how long it might take to stress them, and boredom while doing low intensity.
What is clear to Dr Skiba is that long, low intensity efforts can have a dramatic impact on increases in mitochondria, capillarization, and other determinants of aerobic fitness. However it is difficult to document over the typical short term studies that are done. One good study tracked rowers over 3 decades, another of British elite rowers showed speed increases at LT1 for rowers that trained 100% below LT1, versus rowers that mixed training with 70% below LT1 and 30% above it. With cycling there is evidence that long-term low-intensity training results in performance increases such as reduction in slow component of vo2, and the often mentioned ability for elites to generate high power outputs after long durations.
FWIW my personal opinion has long mirrored what I hear ISM say. Put in some good work (90-120 minutes) at low-intensity and then throw down some hard efforts at the end. Do some sprints, go for some 1-4 minute KOMs, whatever, as long as it doesn’t impact upcoming workouts. If I happen to do a hard effort early on, don’t sweat it, just settle back into low intensity and keep going knowing that I’m accumulating a solid 4-6 hours of steady low-intensity a week, and my belief there is evidence in my own data of a cumulative effect from doing what amounts to endurance maintenance work most of the year.
That’s one science opinion plus my thoughts, there isn’t a lot of endurance coverage in Skiba’s Scientific Training for Endurance Athletes as its pretty simple training. He does suggest adding some tempo into long rides, to reduce boredom and induce more metabolic adaptations. Something like a 4 hour endurance ride, with 3x10-min of tempo, and capping any hills at 95% FTP (he actually uses CP, which is a little higher than FTP).