Canova told everyone this a decade ago. It's maybe even more true today.
This is a good post.
The amount of times that I see people suggest that there is a "no man's land" between easy pace and threshold pace, I have to roll my eyes. The most successful coaches in the world (particularly at the marathon, but even Dave Smith with 8k/10k) do a ton of sub-threshold work around marathon pace.
Kipchoge's logs are basically 5% or less at 10K or faster, about 20% in the HMP-MP range, long runs that 15-30s per KM slower than marathon pace, and then tons of mileage. They religiously keep that 10K and faster work so low specifically because they know they need to run the mileage and the body cannot handle both.
And who was it that started to promote the idea of "grey zone intensities"? Jack Daniels.
He also wrote that it is enough with 20 minutes of LT running each week...