Odd concept, but true. Many of "the fast hikers" are rather more accurately called "long day hikers". When you hike from just after sunrise to just after sunset at a leisurely 2 MPH, it is very easy to get in 20 MPD.
As Jack said, if you don't spend a lot of time in towns, and mainly hike all day, it is very easy to do a "fast" 4 month hike on the AT.
Nothing superhuman. Hiking all day is not everyone's taste. But some us just like the simple act of walking all day.
I know I do.