Evolution via natural selection happens because our genes are passed forward, the main reason we evolved to this point is because the ones who could not survive the pre-modernised world did not pass on such genes, because in the modern world with medicine et cetera, much less people die and thus our evolution has flatlined, because everyone has a chance at passing on their genes. Excluding artifical attempts at altering the populations genology, chances are it will remain the same.
Most humans do not live sedentary lives now. We are out on community leagues playing softball. We run to deal with the stresses of heavy workloads. I can't find anyone who does not own a gym membership. Most adults are trying to be more active these days. So many go camping and hiking regularly. So the baton will be passed to the next great distance runner, one way or another.
We have inherited an ability to run for long distances from our ancestors, who needed to chase prey and escape from prey over long distances. However, with so many people living sedentary life styles and the need to run for survival disappearing in most cultures, this fitness gene will only be handed down by fitness fanatics who are in the minority.
Evolution happens incredibly slowly over thousands of years so it would be difficult for us to see it happen in our lifetimes anyway. But I expect there to always be people pushing themselves to the absolute limit. Lots of children follow in their parents footsteps in some way so because of this I wouldn't think our running capabilities will change.
Evolution is a function of environment, with nature favoring those traits that help a species adapt and survive. Human beings no longer require the ability to run long distances to survive in a world of smart phones and desk jobs. Other traits with more pressing, immediate needs will take evolutionary priority over distance running.