I doubt this is the same thing, but in Jr High, I was complaining that my side hurt doing the mile run and the coach had me breathe through my nose instead of my mouth; sure enough the pain went away.

That's a very interesting idea. I've always breathed through my mouth while running due to not getting "enough" air through my nose, and also fight with abdominal pains whilst running as well. I'll give this a shot tonight actually, in theory it seems very similar to any other kind of resistance training.

Apparently yes, nasal breathing is hugely beneficial. This book was a life changer for me — James Nestor’s Breath:

Breath: The New Science of a Lost Art: Nestor, James: 9780735213616: Amazon.com: Books Breath: The New Science of a Lost Art: Nestor, James: 9780735213616: Amazon.com: Books https://www.amazon.com/dp/0735213615

Yes, yes, we all know Pranayama and other ancient sciences already “covered” it, but at least for me, I was totally unaware of the importance and wide ranging benefits of nasal breathing, until I read this book.

For me the key take away was: when running, follow 2 simple rules: always breath through your nose, And make your out breath last (much) longer than your in breaths. I started with 2 steps for out, 3 for in, then 2/5, And now i can do 2/7. I use the nasal breathing rule as a guideline to know when I’m running too fast - if you can’t just breath through your nose then you’re running too fast. Over a few weeks of running 3 times/week I was able to increase my distance from 2 to 5 miles.