I'm not saying they did not make this offer, but I hadn't heard of it before. Usually they will charge you a price to reinstate miles. Have they actually expired yet? If they haven't expired and you need some transaction to keep them active, there are many other ways than taking a flight.

Also, does the flight have to be on US metal, or can it be on another carrier and credited to US?

If you really want to be cheap, you might be able to do something like take the train to PHL, fly PHL-RDU, then take the train from RDU back to DC. (I'll give you a ride from RDU to the train station here if you need it!)