According to most Poskim, it takes 18 minutes for an average person to walk a distance of one mil, and therefore 72 minutes to walk 4 milin.… Others say it takes 22.5 minutes to walk a mil, and therefore 90 minutes to walk 4 milin.
(Emphasis in the original.)
If it takes me 18 minutes to walk a distance, it's going to take me more than 72 minutes to walk four times that distance. People walk far distances more slowly than short distances as far as I can tell.
More extremely, I see the same proportion applied to four amos, which is even more unrealistic. Surely if it takes me 18 minutes to walk a mil (2000 amos), it doesn't take me 18 ÷ 500 minutes to walk four amos! I don't walk a mil at anywhere near the speed I walk four amos at!
Have any pos'kim dealt with the idea that those numbers don't match up?
Edit: Double AA pointed out in a comment that I'm asking this backward: the original source says it takes twelve hours to walk forty mil, and the other distance-time conversions are derived from that one. Either way, the question remains.