[thras]

And by listing negative effects, you can also make bad things sound bad.

Lots of people kill themselves with marathons. Maybe there's a right way to do it, but obviously it's easy to do it the wrong way. Which makes marathons dangerous until someone figures out your right way.

"if running a marathon is bad, then surely running half a marathon is still not good, right?"

No. Does not follow. As you say, there are probably distances where the good far outweighs the bad. 26 miles is not that distance.

[dgordon]

No, "lots of people" don't kill themselves with marathons. A few people who try to go from couch potato to marathon in 12 weeks or who chug ridiculous amounts of water over the course of a 5-hour walk-jog kill themselves with marathons. (It's possible to drink to the point of excessive dilution of the sodium in your bloodstream, which can kill you just like dehydration can, but this is basically impossible to induce below an Ironman triathlon distance if you're reasonable about water intake.) That's not "lots."

Distance running is what humans evolved to do, according to a good deal of evidence. Admittedly, running on asphalt and concrete with awful form created by a lifetime walking and running in awkwardly built shoes and sitting in chairs all day is not what humans evolved to do.