Not much of that running was really hard/fast. Many of those miles were comfortable steady state as fast as they wanted to go at that time. Fast continuous runs like 10 miles easing into it, and then maybe under current half marathon pace for the last half.
It was time efficient running- just get out the door and get your miles in sometimes at a quick but sustainable pace. None of this get in a car to drive to a nicer place to run, warm up easy, stop and take off some clothes/get a drink and a gel, maybe change shoes, do repeats constantly glancing at your GPS watch, change shoes and clothing again to warm down, then drive home.
There were fewer people then and traffic was not as bad, so you had road training loops you could do directly from your home without risking injury or death.
A good fit runner could sustainably average over 120 miles a week like that devoting 15 hours or less to running every week.
There were no cell phones, no home computers, no cable TV, no internet. network tv was limited, few distractions.