And by listing negative effects, you can also make <i>bad</i> things sound bad.<p>Lots of people kill themselves with marathons. Maybe there's a <i>right</i> 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.<p>"if running a marathon is bad, then surely running half a marathon is still not good, right?"<p>No. Does not follow. As you say, there are probably distances where the good far outweighs the bad. 26 miles is not that distance.