I rarely run over 3.1 miles. Even then a 10k is about my limit. This is mostly due to the fact that running over 30 minutes or so tends to start mentally boring me. I have read for many years that chronic long distance training has numerous downsides to include chronic inflammation and muscle atropy. One of the images that always stuck in my mind was the visual difference between a sprinter and a marathon runner. Many would assume at first mention that the marathon runner and sprinter represent ideal healthy individuals but when you look at the makeup of their bodies, ask yourself..who are you trying to look like?
For more info check out the article: The 5K, Not The Marathon, Is The Ideal Race | FiveThirtyEight