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
Risk of stroke is the most obvious (and fairly concerning) one where there's solid data.
Also there's "distance running" as in running maybe 50k a week, that's probably okay, although as you get older it will increase your risk of stroke. But ultramarathons are a whole different ball game and almost certainly bad for you
Can't remember where I heard it-- some MD on a podcast recently-- but he mentioned distance running increases visceral fat in your muscles and around your organs when compared to HIIT and weight training, and visceral fat is generally a health risk indicator.
Why wouldn't it be, you think the body was made to run all day? Most animals lay around all day with some bursts of activity. The exercise fad is just a peculiar neuroticism modern Westerners have.