Who said it was hard? Most just don't want to commit that kind of time to running.
The only people I know who can sustain high mileage + full time job have the following characteristics:
Only need 7 hours of sleep per night
Naturally quick easy pace
No other hobbies
Super understanding spouse
Yes, I don’t think it is hard. Personally, I just don’t have the mental focus to run that much. It’s boring to me. I like workout days. I can’t go over an hour on an easy day.