I just played around quickly and I'm sure a better deal can be found if you fly out of BWI... but I was able to price a DCA-RDU-CLT-PIT-SAN-PHL-DCA itin at around $610 (on 4/6 and return on 4/8). This would log you 6467 miles.
On a secondary note, do you still have to be a Preferred to use your certs? Remember, you can send them in for miles now! Or sav them until you make Silver again after the challenge.