Final answer:
The sentence in question contains a punctuation error, not a run-on sentence. The comma should be replaced with a period, semicolon, or a coordinating conjunction to correctly join the independent clauses.
Step-by-step explanation:
The sentence 'Aimée Marcoud, a trapeze artist, performed under the name Miss Fillis, she retired at the age of fifty-five.' is not a run-on sentence; however, it does contain a punctuation error. It improperly uses a comma to join two independent clauses without a coordinating conjunction.
To correct this error, the comma should be replaced with a period or a semicolon, or a coordinating conjunction could be added. For instance, it can be rewritten as: 'Aimée Marcoud, a trapeze artist, performed under the name Miss Fillis. She retired at the age of fifty-five.' or 'Aimée Marcoud, a trapeze artist, performed under the name Miss Fillis; she retired at the age of fifty-five.' or 'Aimée Marcoud, a trapeze artist, performed under the name Miss Fillis, and she retired at the age of fifty-five.'