I've had this before (had a rental for 4 months!) - what I did was inform them that I needed the car for longer, and told them what the mileage was currently on the vehicle. They were happy with how things were with the vehicle (if they had wanted to exchange the vehicle mileage wise, that wouldn't have been a problem), and all they did was 'roll-over' the rental on the system, so as to overcome the limit of days