In a study by Matthew Wilson, which was published in the Journal of Applied Physiology,
advanced imaging techniques were used to look at the hearts of three groups of people: 12 lifelong 50- to 67-year-old athletes, 20 age-matched control subjects, and 17 younger (26- to 40-year-old) athletes. Heart-muscle scarring was detected in four of the lifelong endurance athletes and was not found in any of those in the control group nor in the younger athletes. So, the association of heart scarring and endurance running is there. But additional research is needed to draw a conclusion that endurance training causes the scarring and is therefore bad for you.
Read more here: http://www.runnersworldonline.com.au/is-distance-running-healthy-for-your-heart/