PLT OF M20 SQUAWKS 1200 WITHIN DC ADIZ.

Narrative

I OBTAINED A COMPUTERIZED BRIEFING VIA DUATS AND FILED AN IFR FLT PLAN FOR GAITHERSBURG; MD. I TOOK OFF FROM 1D2 (PLYMOUTH; MI); AND ACTIVATED THIS PLAN WITH DETROIT APCH CTL. WHILE APCHING THE MARTINSBERG VOR; I ASKED TO CANCEL IFR AND PROCEED VFR DIRECT TO GAITHERSBURG. I WAS TOLD TO REMAIN ON CURRENT HDG TO MARTINSBERG WHILE THE CTLR CHKED ON MIL OP STATUS IN THE ADIZ. A FEW MOMENTS LATER; THE CTLR SAID THE REQUEST WAS GRANTED; AND I SAID I WOULD RETAIN THE CURRENT XPONDER CODE FOR FURTHER ADVISORIES. I BEGAN A VFR DSCNT AT THIS TIME. SOMEWHERE NEAR; OR SHORTLY PAST MARTINSBERG; I STARTED CONCENTRATING MORE CLOSELY ON LCL NAV; AS I HAD NOT BEEN TO THIS ARPT FOR APPROX 4 YRS. TO MINIMIZE THE DISTRS FOR THE LAST PHASE OF THE FLT; AMONG OTHER PREPARATIONS; I SWITCHED THE XPONDER TO 1200; MOSTLY BASED ON FORCE OF HABIT. I LATER SWITCHED TO GAITHERBURG CTAF; OBTAINED THE NECESSARY INFO; AND LANDED WITHOUT INCIDENT. I WAS MET BY A WORKER AT GAITHERSBURG; WHO TOLD ME THAT I WAS TO CALL TRACON AS SOON AS POSSIBLE. I DID SO; AND AT THAT TIME; FIRST LEARNED OF MY ERROR IN PROC.