COMMANDER 112TC PLT INADVERTENTLY TAXIES ON THE ACTIVE RWY AT MYF.

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I AM A PARTNER IN A ROCKWELL COMMANDER HANGARED AT MONTGOMERY FIELD IN SAN DIEGO. MY PARTNER TAXIED THE ACFT TO THE MAINT FACILITY. SINCE HE WAS NOT AVAILABLE AND THE MAINT WAS COMPLETE; I DECIDED TO TAXI THE ACFT BACK TO OUR HANGAR. I BEGAN TAXIING THE PLANE BACK FROM THE MAINT FACILITY. I STARTED THE ENG; TAXIED TO THE BOUNDARY OF TXWY I (AT TXWY G). I CALLED MONTGOMERY GND CTL FOR A TAXI CLRNC. GND CLRED ME TO TAXI TO PARKING. I TURNED R ONTO TXWY G AND MISSED THE TURN ONTO TXWY H AND PROCEEDED R ONTO RWY 10R; WHICH PARALLELS TXWY H. LOOKING DOWN THE RWY; I SAW A CESSNA ON SHORT FINAL TRANSITIONING TO A GAR. GND IMMEDIATELY CONTACTED ME AND ASKED ME WHERE I WAS GOING. I MADE A MISTAKE AND TOLD HIM SO. GND TOLD ME TO TURN AROUND AND EXIT THE RWY; WHICH I DID. THEN HE CLRED ME TO TAXI TO PARKING ON TXWY H. GND ASKED IF I WAS A LICENSED PLT. I SAID I WAS. AFTER I REACHED TXWY J; GND INSTRUCTED ME TO CONTACT THE TWR. I HAVE HAD A PLT'S CERTIFICATE SINCE JAN/54. MY MEDICAL EXPIRED IN 1995 AND SINCE THAT DATE I HAVE NEVER FLOWN WITHOUT ANOTHER LICENSED PLT ACTING AS PIC.