Snell did not have anyone do anything. It was Arthur Lydiard who had Magee and co., do long runs.
Snell did become a noted exercise physiologist. When asked what he would change about his training, looking back, he said, "very little."
There are examples of outliers at both ends though. Sure, there are the athletes who manage 100-120 kms per week and somehow race well internationally. Then there are the Mo Farah and Cameron Levins of the world who would run 160 - 180 kms per week for Farah and 200 -plus for Levins. Their fastest performances are in the 2000m distance not the 5000m/10,000m/marathon that they competed in.
Farah: 3:28.81 in the 1500 and 8:03.40 in the two mile.
Levins: 8:14.69 in the two mile and 4:55.35 in the 2000m.
Neither competed at that distance regularly.
The cardiac efficiency allowed them to take in and utilize oxygen better, even at lactate-spewing efforts.
Sifan Hassan's third best event is the 1500, which is a European record. She trains
around 120 miles (193 km) per week. She holds 11 national or European records from the 1000m distance to the marathon - another athlete with great range.
Walker was up to 110 miles per week. Long before the super shoes he ran 3:32/3:49.
Ingebrigtsen is an amazing talent, however, has been outkicked in the 1500m by Brits who run more volume.