I have a few records:
Most miles in shortest time: During the power outage in Atlanta, I drove a rental car 800 miles in 14 hours from Florida, handed the keys to the desk agent at the airport in Charlotte and said “thanks for getting me home when Delta let me down. Again.”
Most over due was a Cadillac Escalade that I happened to really, really like. It was five weeks over due. I started to get the threatening letters when it was about three weeks over due. Sorry Hertz.
Most miles on a single car was about 7000. Depending on where I am, I’ll keep the cars until they need major maintenance — sometimes even just leave them at the terminal for a few days until I come back to the city.