B757 INADVERTENTLY ENTERS A RWY AFTER BECOMING CONFUSED WITH HIS TAXI INSTRUCTIONS.

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THERE WAS A NEED TO FERRY A B757 TO AUA; ESTIMATED TIME OF DEP XA00 AND RETURN WITH A PLANELOAD. THE ACFT WAS LOCATED AT THE HANGAR AND AFTER COORD WE GOT THE ACFT OFF THE GND A FEW MINS LATE. UPON RETURNING AND LNDG OVER 6 HRS LATER; WE WERE NOTIFIED OF OUR GATE; SO WE OPTED FOR RWY 9L; AND MADE AN UNEVENTFUL LNDG. FATIGUE WAS REALLY SETTING IN AT THIS POINT. I HAD ALREADY LOOKED ON MY 10-9B CHART FOR THE LOCATION OF GATE AND OUR EXIT FROM THE RWY (TXWY M5) HAD US HEADED STRAIGHT FOR SPOT X -- OUR ENTRY. HAVING ENDED TRIPS RECENTLY IN THE CONCOURSE; AND HAVING A FATIGUE RELATED MENTAL LAPSE; I TURNED THE ACFT EBOUND ON THE RAMP (TXWY N) BEFORE REACHING SPOT X; THINKING OF GOING TO GATE. AFTER HAVING TAXIED ABEAM GATES AND REALIZING WE'RE GOING THE WRONG DIRECTION; WE TOLD GND WE'D HAVE TO GO BACK FOR SPOT X. THEY TOLD US TO EITHER DO A 180 DEG TURN ON THE RAMP; OR TAXI BACK ON TXWY M TO TXWY M5 TO RE-ENTER. AS I TURNED THE ACFT NBOUND ON TXWY M6; A SUPER 80 WAS STOPPED ON TXWY M EBOUND ABEAM GATE XX. I THOUGHT THE SUPER 80 WAS ON TXWY N; BUT IN FACT HE WAS ON TXWY M; AND HAVING BEEN TOLD TO DOUBLE BACK ON TXWY M. I JUST KNEW THAT MIA GND WAS HAVING ME ON MY WAY IN A EXPEDITIOUS MANNER. THINKING I HAD JUST EXITED THE RAMP (REALLY ON TXWY N) AND SEEING THE SUPER 80 HOLDING ON TXWY M (THINKING HE WAS ON TXWY N) THE ONLY LOGICAL PIECE OF E/W CONCRETE AHEAD OF ME WOULD BE TXWY M (NOT SO -- IT WAS RWY 9L) OUR MOMENTUM WAS CARRYING US ALONG WHEN I SAW THE RWY HOLD LINES; BUT NOT IN TIME TO MAKE A COMPLETE STOP. I'D SAY THAT WE WERE IN A COMPROMISING POS WHEN MIAMI GND TOLD US THAT WE WERE ENTERING THE RWY; AND TO CONTINUE ON IT WBOUND TO TXWY M5 FOR OUR ENTRY POINT. THEY DIDN'T SEEM TOO UPSET BY OUR 'PROGRESSIVE TAXI' MODE; BUT I CERTAINLY WAS. THIS SIT WAS AN ACCUMULATION OF FACTORS SUCH AS FATIGUE; POOR COMS BTWN THE COCKPIT CREW MEMBERS AND MIA GND; OPS IN AN AREA OF THE RPT WHERE TXWY N AND THE RAMP BLEND; AND TRYING TO GET OUR PAX ON THEIR WAY. ALTHOUGH FAMILIAR WITH MIA ARPT; HAD WE BEEN MORE CAUTIOUS AND WATCHED OUR DIAGRAMS ON THE 10-9 CHARTS; THIS OCCURRENCE WOULD HAVE NEVER HAPPENED. SUPPLEMENTAL INFO FROM ACN 523752: WE CLRED THE RWY AND WHILE I WAS PERFORMING THE AFTER LNDG CHKLIST THE CAPT TAXIED PAST OUR ENTRY SPOT. WE REALIZED IT AND THE CAPT REQUESTED CLRNC TO TURN AROUND. ATC ADVISED HIM TO MAKE A L TURN ON TXWY M OR A 180 DEG TURN. THE CAPT CHOSE THE L TURN. I SAID TO HIM WE CANNOT MAKE THAT TURN SINCE A SUPER 80 WAS THERE. HE SAID THE SUPER 80 WAS HOLDING. I STARTED TO QUESTION WHETHER OR NOT I HAD HEARD THE CLRNC CORRECTLY SINCE HE ACKNOWLEDGE WHAT I WAS SAYING TO HIM. THERE WAS A TURBOPROP ON THE RWY BACK TRACKING SO I REALIZE WE WERE NOT IN ANY DANGER AS WE PROCEEDED TOWARD THE RWY. GND ADVISED US THAT WE WERE ENTERING THE RWY AND HE STOPPED THE ACFT SUDDENLY. SINCE NO OTHER ACFT WAS IN POS FOR TKOF; GND ALLOWED US TO BACK TRACK.