It turns out that running, pretty much like everything else in life, may be best for you when taken in moderation.
A review article published by James H. O'Keefe and others in the June 2012 issue of Mayo Clinic Proceedings suggests that the type of chronic, high-intensity exercise that marathoners, ultramarathoners, triathletes, and long-distance cyclists participate in damages the heart (article here, review here). This damage, termed cardiac remodelling, is caused by repeatedly making the heart work really hard for really long periods of time. The result is myocardial fibrosis, arterial calcification, stiffening of the arterial walls, and a reduction in diastolic function. Yikes!
The authors don't suggest running is bad. In fact, the opening sentence of their article touts running as a cornerstone for cardiovascular and overall health. They also are quick to point out that these adverse effects from high intensity exercise are an untested hypothesis and that inconsistencies in the data do exist.
What does all this mean? Go for a run. There is a large and growing body of evidence that running not only improves the length of your life but the quality of your life. How long you run depends, in part, on why you run in the first place. If your main focus is increased longevity then, according to Dr. O'Keefe, you may be doing more harm than good if you run more than 3 to 4 miles more than a couple of times a week. But how many ultramarathon-type athletes are motivated by cardiovascular health alone?
Personally, cardiovascular health is definitely a perk of running but it hardly gets me out the door, or up that mountain pass, or through the final hours of the second long run of the weekend. There are so many other reasons that I run and that I like to run long in particular. Heck, it takes nearly 3 miles to shake the cobwebs out of my muscles and settle into a comfortable groove. If that's when I stopped I'd surely quit altogether, which would be even worse for my ticker.
I think the jury is still out on whether endurance running is bad for your heart. I don't know how motivated I would be if I knew my efforts were taking years off my life. Time and more data will determine whether that's true. In the mean time, I'll keep running.
UPDATE: A new editorial by the authors (O'Keefe and Lavie. 2012. Run for your life... at a comfortable speed and not too far. Heart doi: 10.1136/heartjnl-2012-302886; reviewed by Outside magazine here) provides some guidelines for heart-healthy running. If longevity is your focus they suggest you can improve your cardiovascular health by limiting your running to < 1 - 2 hour per day, < 5 days per week, and at a pace slower than 8 min/mile (5 min/km). Better yet, they suggest you walk, don't run.
The data are still correlative and far from overwhelming. (Never trust a graph without blatant estimates of variance!) There also is no mention of potential differences in the quality of life between exercise regimes. The authors admit their exercise guidelines are aimed at maximizing heart health and not heart fitness and aerobic capacity, which may be important characteristics of a high quality of life for some people. They also make no mention of the many other physical and mental benefits associated with running and how they might vary with intensity.
Ultimately, how long you run will be determined by your reasons for running. If you run strictly for your health it's unlikely that your exercise habits would be classified as excessive anyway and you're probably happy to learn that you can gain the same benefits or better with less effort. If you run because you like to run and you enjoy a good, strenuous effort on a regular basis you may decide that's not worth giving up for some benefit that you may or may not realize in your twilight years. Even O'Keefe admits that "driving to [a] marathon is a hell of a lot more dangerous than running in it".
Now someone remind me, will this cup of coffee enhance my performance or hinder it?
