Escalation paths in this situation:

1) Ask to speak to the station manager. The quality of agents at HLEs is...variable. Managers should, in theory, be a little more motivated to help you out. Some HLEs don't have dedicated on-site managers, though.
2) Ask the agent to call the Area Manager. Area Managers are responsible for directing the movement of cars between locations. They can have the shuttle crew bring a car or two out from the airport or another location to yours, or they can arrange to transfer your reservation (and you) to another branch with cars.
3) Tell them that if they cannot provide you a car in or close to your reserved class, then they are obligated to give you the next larger thing they have (at no extra cost), and you will gladly come back to switch to the smaller car once they get one in.
4) Take the larger car and drive it to the nearest airport and switch for your reserved class.
4) If they are being uncooperative, call the Hertz web support helpdesk at 1 (800) 654-2270, option 1, and ask them to transmit a message to the area management team on your behalf to get some results.

Hertz deserves 99.9% of the blame here, but I'm left scratching my head why anyone would allow two days to elapse without making some effort to push Hertz into honoring a confirmed reservation...