it's mostly mental wrote:
It's 20 miles per day or less than 2.5 hours in total work per day. Hard, but not exactly superhuman. Certain HS programs used to have guys as young as Sophomores run 100+ after only a year of running. It's not much of a stretch to get to 140 if you're a mature adult with more running experience. Go talk to athletes in other sports (swimming, cycling, tennis) and 2.5 hours per day doesn't seem unusual or even high.
Well none of those sports come close to the impact that running has on your body. Swimming and cycling are much easier on you, because the impact forces are massively lower, if present at all. Additionally, because of the nature of cycling, it's much more common to go longer- they have to prepare for extremely long races for multiple days in a row. It's not as hard to recover from as running. Example: a friend of mine was a collegiate 800/1500 runner at really low mileage (got injured over 35). Now she rides a ton on the bike. Sometimes 50+ miles a day.
With tennis and other skill-intensive sports the work style is very different. When I was younger I worked pretty hard at both tennis and running. A lot of tennis workouts are isolating one skill at a time and doing different drills, returning balls from the machine, plus some running, and strength work. So the forces and stress on the body are constantly changing (running is pretty much a continuous repeated strain) and there are more periods of low intensity.
Let's not pretend 140 mpw is not a sh*tload. IMO, run 140 mpw if you want to, but you're not getting much you couldn't get from 100 mpw, except an increased risk of injury/burnout.