PLT OF C172 IMPROPERLY SQUAWKS VFR CODE WITHIN DC ADIZ AFTER CANCELING FLT PLAN WITH PCT.

Narrative

I WAS ON THE FINAL LEG OF MY IFR FLT PLAN WHICH ORIGINATED IN COLUMBIA; SC; THAT SAME MORNING WITH A STOP IN RICHMOND; VA. I WAS ABOUT 5 NM FROM MY HOME BASE AT JYO AND; AS REQUIRED; I TOLD THE ATC CTLR THAT I 'HAD THE ARPT IN SIGHT' AND REQUESTED TO HAVE MY IFR FLT PLAN CANCELED. THE CTLR TOLD ME THIS WAS DONE AND GAVE ME PERMISSION TO 'CHANGE TO THE ADVISORY FREQ.' I ALSO INADVERTENTLY INTERPED THIS AS PERMISSION TO LEAVE MY ASSIGNED SQUAWK CODE AND CHANGE TO THE VFR SQUAWK CODE 1200. DURING THE APPROX 5 MINS FROM THIS TIME TO LNDG; I PROCEEDED DIRECTLY TO THE LEESBURG ARPT AND LANDED PROMPTLY; SECURING MY ACFT DUE TO THE GUSTY CONDITIONS AND ICE AND SNOW ON THE RAMP. AT THE TIME OF LNDG; I WAS GIVEN A MESSAGE TO CALL A PHONE NUMBER AND WAS INFORMED OF THE SQUAWK CODE DISCREPANCY. IN RETROSPECT; I THINK WHAT HAPPENED WAS THAT DURING THE VERY LAST FEW MINS OF A LONG AND ARDUOUS DAY OF SOLO FLYING THE CUMULATIVE FATIGUE AND STRESS OF THE EFFORT CAUGHT UP WITH ME AND I INADVERTENTLY WENT BACK IN TIME TO THE OLD PROC I HAD BEEN RECONDITIONED TO DURING THE LARGER PART OF MY TRIP; SELECTING THE INCORRECT SQUAWK CODE FOR A VERY BRIEF PERIOD OF TIME. IT WAS AN HONEST MISTAKE I VERY MUCH REGRET; BUT NO ONE WAS HURT; NO PROPERTY WAS PUT IN DANGER AND I CERTAINLY HAVE LEARNED MY LESSON.