ultra runner here...I've never heard this before. can you provide sources for "considerable evidence"?
At what point did running really far become the thing to aspire too when getting fit?
What about strength? Speed? Flexibility? Agility? Regardless of the evidence, most people are better served doing a good mix of activities and being generalists.
The only reason to run more than 10km is if you like running. Otherwise 5-10km at a decent pace is just a good cheap way to get effective cardio. You should probably stop there and lift something heavy.
For example, a recent study found that two thirds of pro cyclists have markedly low bone density: https://www.velonews.com/news/dutch-study-two-thirds-of-pro-....
You could at least make a good faith attempt and meet people at least halfway though, right?
FWIW I've read studies saying the same thing- my critique is just with your approach here. You wouldn't believe someone who just waltzed in with a statement and refused to post any references, would you?
This is, in fact, exactly how you convince people.
So will 20 lbs of lower muscle mass which I can gain through...you guessed it, running.