Technically, a run-on sentence is two complete sentences that are improperly joined. For instance,
I like to write I do it all day. (Negative example)
If you stick a comma between them with no coordinating conjunction, you're made a comma splice, and that's wrong too:
I like to write, I do it all day. (Negative example)
You can fix it with a semicolon or a comma and coordinating conjunction, of which there are only seven (and, but, or, nor, for, so, and yet).
I like to write; I do it all day. (Correct example)
I like to write, and I do it all day. (Correct example)
You can also subordinate one of the two:
Because I like to write, I do it all day. (And don't get on me about starting a sentence with "because." It's a perfectly good subordinating conjunction.)
However, there's a good chance that when someone tells you your sentences are run-on, what they mean is that they're too long to follow easily. The sentences may be technically correct, but the reader can't track them. So take a look at what you have and see if you can tell what your critter meant.