I, too, had been staying at the Tech Center in Marriott in Denver but, six weeks ago, switched my loyalty to the Gateway Marriott near DIA.
I didn't have problems like yours, but I can guarantee I would have been less than happy if the manager had told me I needed to park my car two blocks away, at night, and walk back to the hotel, as a female alone.
This is unacceptable. Ask for some sort of retribution and let us know how they handle it.
I am hoping they handle it well, because I really love Marriott!