I was a little curious, so I did some searching, and ran across a study about damage from long runs.[1] You may be aware of that already.

I did eventually come across some stuff about a "dangerous lung disease" marathon runners are at risk of.[2][3] The news article seems a little scare-mongery (no surprise), but it appears to be backed by science, from the little I read into it.

1: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4415847/

2: https://www.express.co.uk/news/uk/343996/Running-marathons-c...

3: https://err.ersjournals.com/content/23/134/416

There's plenty of exercise science stuff, much of it conflicting. Also, even if some damage to health from running were 100% certain, you need to get a handle on just how likely and how dangerous the damage is. Is it analogous to exposure to a carcinogen that has a 95% chance of leading to cancer and death within 2 years? Or is it analogous to exposure to minor carcinogen that means you have, say, a 2% greater likelihood of developing cancer over the course of your life?

For me, I see that I and all my running friends seem healthier and more vibrant than an average person of the same age. My heart is stronger, running improves bone density, (at least for me) improves strength and reduces pain in joints, improves my balance, helps keep me slim, etc. It gets me out every day, often into beautiful mountains with friends. For as long as I keep running I expect to reap these benefits.

Would I stop or cut down if I were told, with certainty, that running would cause me to die, say, 2 years sooner than I would otherwise if I wasn't running? Not likely, it increases my quality of life in too many important ways. If that number were instead, say, 10 - 20 years, then yeah sure, I might cut down. But there is no science remotely resembling that. I don't think there's anything out there that establishes to any degree of confidence that lots of running reduces life expectancy at all.

Micah True in 2012. Also note that for marathons the number one cause of death is cardiac arrest. Now that is not explicitly what OP said, but you can see how one could draw a valid conclusion from that observation. I don't think that is "baseless", but I would agree that it is not necessarily a smoking gun. https://en.wikipedia.org/wiki/List_of_marathon_fatalities

Interesting, given that I was down in Copper Canyons just a few weeks ago to run the Caballo Blanco race. I don't see Micah True as establishing anything. Yes, it's likely he died of cardiac arrest. Yes, he displayed the enlarged left ventricle characteristic of people who run a lot. But as far as I'm aware, his family history is unknown. It could be that he died of a genetic (inherited) disorder and that he actually lived longer, delayed his heart attack, _because_ he was a runner. (This is the likely situation with Jim Fixx, a famous guy back in the 80's who promoted the health benefits of running, but then died of a heart attack: https://en.wikipedia.org/wiki/Jim_Fixx#Death ) One current conjecture in medical science is that enlarged left ventricle and scarring increases likelihood of cardiac event; at this point I'm pretty sure it remains mere conjecture.

Regarding marathons: what would you expect to be leading cause of death in marathons? Surely not cancer. Running a marathon stresses your heart. If you have genetic cardiac issues (which many do) it's more likely that they will appear in a marathon than when you're walking down the street. It's a huge leap to suggest that training for a marathon increases overall risk of heart failure. It could just as easily be that the marathoner has actually extended his/her life by running, that they would have died earlier if they hadn't kept their diseased heart as healthy as it could be by doing lots of running. As far as I'm aware, there's no science out there that establishes that marathoning causes people to die earlier than they otherwise would, and lots of evidence to the contrary.

Hyponatremia (broadly low sodium levels from drinking too much water) is one of the leading causes of death during/shortly after a marathon.