While I was at Naval Recruit Training Command and undergoing the rigerious PT needed to transform a fat NEET into a sailor, I came to the realization that, realisticaly, Sakaki's running ability could only about through some dedication to exercise. This is all the more obvious when you remember that she consistently outperforms someone who is a very dedicated athlete, Kagura.
Even if she really does have a lot of natural ability, she must still run quite a bit to be that good, even if not as much as Kaguya. I can only assume I didn't realize this before since I'm used to accepting the uselessness of hard-work present in a lot of media and much of our culture.
What does it change about Sakaki's character to know that she apparently exercises a lot, but does so alone, even though one of her friends would make a natural running partner?