Galen's arguable fastest effort compared to his potential is his 26:44 10k of course there are the rumors that he often ran 5:30 pace on his easy days thats about 128% of his 10k pace a 38 minute 10k athlete would be running more like 7:49 per mile at the same percentage that's near the fast end of the range Jack Daniels recommends (7:37-8:24) and considering Daniel's E pace is faster than some people like I would say that its relatively "fast" for Galen certainly not slow Salazar has gone on record multiple times about how he doesnt believe in just slogging out miles one of the first things he did with Mo Farah is get him to increase his easy day pace
GW is right in this sense for new runners you get benefit from any kind of running but once you get to a higher level easy runs dont have the same benefit as they once did so often the pace is increased