That might help, but I wouldn't define a run-on sentence by whether or not you need to take a breath in the middle of it. I define a run-on sentence as two or more complete thoughts put together without proper punctuation or conjunctions.
Here is an example of a run-on sentence:
"I went to my friend's house, we had fun."
This is a run-on sentence because "I went to my friend's house" and "we had fun" are both complete thoughts, i.e. having both a subject and a predicate, but they are only separated with a comma, which is grammatically incorrect. You could correct this run-on sentence in multiple ways:
... with a conjunction (e.g. "and," "but," "so"):
"I went to my friend's house, and we had fun."
... with a period:
"I went to my friend's house. We had fun."
... or with a semicolon:
"I went to my friend's house; we had fun." Note that semicolons are used to indicate a connection between two thoughts, so don't use them willy-nilly in place of any old period.
I hope this helps. Ultimately, the breath thing might be a useful writing tip, but it alone does not define a run-on sentence. You can have a long-winded sentence with a lot of clauses, but as long as its clauses are linked together with the proper grammar, it is not a run-on sentence.