When I was a neophyte, I read one of running's great coaches - Lydiard or Daniels - define "long run" as training runs at or over 17 miles. I thought this was an incredible vanity....and an impossibility.
I understand why he reserved the term "long" for runs in or above the high teens. Humans can store enough easily consumed carbohydrates to run 15 to 20 miles. Beyond, we need to consume a blend of carbohydrates and fat. By 17 miles, your body is either well into consuming that blend, or you've hit the wall.
I flirted with the wall today, consciously throttling back my pace to avoid it. Its my longest run since May. The faster you run, the greater the carb to fat consumption ratio becomes. So I trundled along.
I'm a minute a mile slower than I was this spring.
What I am about is making training for the marathon a lifestyle. Contemporary medical advice recommends 60 - 90 minutes a day of vigorous exercise. This is consistent with training for the marathon at the amateur level.