Here's a comparison and contrast about marathons and mud runs right now as I see it:
Both are hard to most people. Both are considered an accomplishment to finish for most people.
However, the word marathon is synonymous with grueling event. People at least train for marathons. Maybe not like so many LRC posters, but they train. Virtually no one goes into a marathon on a whim. We see plenty of hospitalizations in running and a few heat and dehydration related deaths in running every now and then. Marathoning has gained in popularity among the regular Joe crowd, so these numbers are going up.
The mud runs have grown in popularity exponentially it seems. The marketing is different. The mentality is different. It's for "tough guys" and all that macho BS. We are currently seeing more hospitalizations from dehydration and overexertion causing rhabdomyolysis from mud run participants than marathoners. This is because a bunch of late 30s-early 40s long former "tough guys" are going out on a whim and pushing themselves like they were 21 again, and reaping the consequences. These guys aren't respecting the event even though none of them would even consider running a marathon without properly training for it.
Now, I'm not saying a tough mudder is harder than a marathon, because I don't think it is. I know plenty of fit people who do them and do fairly well in the fields but have struggled mightily with marathons even with proper training. But, from the perspective of hospitalizations and visible popularity, the appearance is that the mudders must be harder.