We run a hell of a lot more farther than we did in the past. It is simply the illusion of modern society and its creation of peaceful working class citizens that makes you think we've lost the ability.
Elsewhere, in the dirt and sand, a Marine is likely trekking mile after mile, farther than his ancestors ever have while carrying over 160lbs of gear.
Modern nutrition and exercise plans allow us to reach greater physical heights than our ancestors, but there is simply less of a need for it depending on what you do.
If you're training for a sport, joining the military or law enforcement. Congratulations, you've already likely surpassed the average endurance of your ancestors. Your superior nutrition and time spent training allows you to be better than them.