When I was a kid, I learned a run-on sentence was a sentence without adequate conjunctions or punctuations to mark and separate the clauses. E.g.: "My wife and I went to a concert we saw The Cure they were terrific." I still have a tendency to write long sentences, but sometimes when I go overboard (e.g., a whole paragraph turns out to be one long sentence) I might break it in two, for clarity. But I don't go to grug-speak extremes.

I think the preference for short sentences in today's prose is a lot like vocal fry among North American women: a deliberate attempt to sound young.