What you're zeroing in on here is the difference between a "plotser" and a "pantser." The former, and I think most writers here in the AH are "plotsers", are those writers who plan out the nuts and bolts of their storiess before writing: outlines, character sheets, timelines, the broad direction of the story. "Pantsers," on the other hand (and I'm one) just start writing without a plan or a plot, and see what happens.
Neither approach is "better" than the other, there's no right or wrong, and if you're extremely one you're definitely not the other (my plotlines can turn on a sentence, and a character arrive just like that). It's useful to know which you are, it stops insanity later.
I agree with your assessment. I am definitely a pantser. I start with a couple of, not even scenes, but a thought of something exciting that I want to explore. I work forwards fleshing it out (I agree completely with Roger Ebert that the muse only visits when you force yourself to write) and backwards trying to create a logical sequence to get where I'm going. The biggest difficulty is inserting a new character, or significantly changing an existing character. There's always a cascade of changes that ripple forward and must be tracked down and fixed. I always fail.
Do you have criteria of when you feel your story is complete?
With this story, the swim meet, I felt the conclusion I wrote finished the arc of the characters at this point. I could have made (and did in different, in-progress versions) slightly different choices that would have extended the conclusion by another week or so, story time, but I would have to come up with more stuff to fill that time and didn't see any point.
I'm 12K words into another story involving most of the same characters, but from an entirely unmentioned character's POV, and set in the weeks immediately after. But that's a whole different thing. I've also fallen in love with a couple of the minor characters and want to write something about them from their POV. So I'll soon have to write up some character lists just to keep track of everything that's happening. That's something I've already had to do for another group of stories that popped up while writing my life drawing model, exhibitionist stuff. It never ends, and I'm closing in on 70. I probably won't make Herman Wouk's 103 years, so I better get to it.