Sometimes they create music at a particular length because that's what worked well on radio, or TikTok, or for a club set. I would go so far as to say that song length is probably chosen for some specific non-artistic constraint more often than it is chosen for some reason intrinsic to the art. So changing the music to accommodate your own constraint, i.e. "this is the part I like when I'm working out and wish it was longer" seems completely reasonable, and isn't even justified in being considered some sacrilegious act.