I get winded walking to the refrigerator! lol Long distance running requires endurance and is a form of continuous running over at least 5 miles. Do you enjoy running?
Running is definitely not my thing.  I do like walking though - put my music on and just lose myself for a bit.
It is a tough sport! Physiologically it is mostly aerobic in nature and you need stamina as well as a lot of mental toughness. 
And good knees!  I know a few runners and they always seem to have issues with their knees due to it being high impact.
I would think the knees would definitely suffer but it's definitely good for the heart as it is used as a means to improve cardiovascular health.