Pretty sure there is not a 62 day limit, you can rent for two, three, or more months.

Either way, not returning it without telling them is probably a very bad idea. They may report it stolen and that's probably not a position you want to be in when the flashing lights come on behind what the police think is a stolen car.

I would first find out if the 62 day limit is real or not, and if necessary drop it off and rent another one, but don't just keep it without telling them.