In my opinion, gut microbiome stuff is massively overhyped.

Here's a study that tried fecal transplants to treat mental illness (and found no effect): https://pubmed.ncbi.nlm.nih.gov/41785480/

The pattern with this stuff is that, when a blinded study is carried out, there's usually no effect.

Visceral fat is fat around organs. It doesn't exist in muscles. I highly doubt that distance running increases visceral fat, that sounds made up. It may be that weight lifting is better for losing visceral fat, but that doesn't mean distance running is bad.

Can you link the data? I don't buy it.

https://pmc.ncbi.nlm.nih.gov/articles/PMC11720530/ says there's a higher rate of AF in older, male althetes but a lower risk of stroke compared to similar aged, non-athletes.

At any rate, the data doesn't seem clear enough to claim that "distance running bad for you" or "any distance over 50K is bad for you".

Claiming that distance running is unhealthy is wild. It can lead to injury, and there's such thing as overtraining. But to claim that it's broadly unhealthy is just wrong