It’s hardly surprising articles in Runners World magazine (for runners) would not support the thesis that too much running could be bad for you. And the study Eric posted of “Relationship of Running Per Week” study is quite old, published in 1997. Furthermore, that paper used cholesterol and other CHD risk factors as defined in 1997 (again pretty old), and not actual mortality statistics.
I was curious so found this article from 2015 that seems to be one of the sources of the data that Runners World is referring to: Dose of jogging and long-term mortality: the Copenhagen City Heart Study
The abstract states: “The findings suggest a U-shaped association between all-cause mortality and dose of jogging as calibrated by pace, quantity, and frequency of jogging. Light and moderate joggers have lower mortality than sedentary nonjoggers, whereas strenuous joggers have a mortality rate not statistically different from that of the sedentary group”. It includes this chart:
So there is some seemingly valid data to indicate that it is possible to overdo running (just as it is possible to overdo everything in life it seems). On the other hand the number of “strenuous joggers” is pretty small, and the observed results may be an artifact of that small sample size.
Furthermore, there are clearly good benefits to exercise of all kinds, especially for us diabetics. And there is no “hard limit” like 20 miles a week that is considered too much - the paper has rather flexible definitions of light, moderate, and strenuous jogger categories that I don’t think many if any of the readers on this site are likely to exceed.
So personally, I do exercise every week and have since I was first diagnosed with T1. I run, use a rowing machine, play squash, shovel snow, strenuous yard work, whatever depending on the weather, and feel better when I do and have seen only advantages. The benefits of exercise are clear from their chart by comparing light and moderate exercisers to Sedentary subjects.
My takeaway is that something like spending decades training for competitive ultramarathons may not be the best thing for you - OK, that could shorten your lifespan. But that kind of training is usually self-limiting because of injuries anyway, not to mention the crazy time commitment necessary. Once again, all things in moderation (like your Mom used to say) seems the best advice. In my opinion running a few times a week, and not to the point where you are injuring yourself or you need to force yourself out the door, would be included in the definition of moderation.