Nobby has posted plenty of info on Japanese methods over the years.
I did get to see a squad of elite Japanese marathoners train in NZ in the 80s.
The premier runners were running 3 times a day, most days.
Most of the running was indeed slow, 5min per km pace. They would often get in 3hrs of this in the day.
Every few days they would do harder sessions. 40kms in 2.10-15 or reps such as 3x5k in the 14.50s. It was the off season and 2 of the runners did a run of 126kms at 4min per km. Those 2 were very experienced 2.8/9 men, one came 4th at the next Olympics. The other guys didn't try anything like that as they weren't thought to be ready for such a task.
My understanding was 250-300kms per week was typical in this phase of their training.