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.