A sentence run-on is NOT a sentence that is too long. Run-ons can be very short:
I worked all night, it wasn’t fun. = sentence run-on
A sentence run-on is when 2+ independent clauses are linked together incorrectly.
Here are the 4 ways to fix sentence run-ons:
1. Switch the comma (or lack of comma) with a semicolon ( ; )
I worked all night; it wasn’t fun.
Notice that the start of the second independent clause is not capitalized.
2. Leave the comma (or add one if it's missing) and add a coordinating conjunction (FANBOYS).
I worked all night, and it wasn’t fun.
3. Rearrange the sentence to make the clauses agree, usually by making an ICDC or a DC, IC.
It wasn’t fun when I worked all night.
When I worked all night, it wasn’t fun.
4. Just separate the clauses with a period (making two sentences instead of one).
I worked all night. It wasn’t fun.