CAPT OF AN MD88 BECAME CONFUSED DURING TAXI FOR TKOF AND EXITED THE ACTIVE RWY AREA AFTER BEING CLRED FOR TKOF.

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ACFT WAS CLRED FROM RAMP TO TAXI TO RWY 22L VIA TXWY A; TXWY A18; AND TXWY D. AFTER TURNING ONTO TXWY D; TWR DIRECTED US TO 'CUT ACROSS THE PAD; CLRED POS AND HOLD RWY 22L; EXPECT NO DELAY ON THE RWY.' FLT ATTENDANTS WERE NOTIFIED AND THE TKOF CHKLIST STARTED. I TAXIED ACROSS THE PAD AREA AND PROCEEDED TOWARD RWY 22L BY THE TXWY DIRECTLY AHEAD OF ME; BELIEVING IT WAS TO THE DEP END OF RWY 22L. AS WE APCHED; TWR STATED; 'CLRED FOR TKOF RWY 22L; ACFT ON 3 MI FINAL.' WHEN THE RWY SURFACE AHEAD CAME INTO VIEW I EXPECTED TO SEE MARKINGS FOR RWY 22L BUT DID NOT. I COULD ONLY SEE THAT I WAS ABOUT TO TAXI ONTO THE RWY I THOUGHT WAS RWY 22L AT A POINT BEYOND THE DEP END AND NO IDEA HOW FAR DOWN IT I WAS. IN ADDITION; THE FACT I HAD NOT SEEN ANY MARKINGS FOR THIS RWY I DID NOT FEEL IT SAFE TO TAXI ONTO IT. FROM THIS POINT; TKOF WAS NOT AN OPTION. THE ACFT WAS NOW SITUATED ON A TXWY PLACING IT DIRECTLY ACROSS THE APCH END OF THE RWY SURFACE TO OUR IMMEDIATE R. I THEN MADE 2 ATTEMPTS TO CONTACT THE TWR BUT WAS UNSUCCESSFUL. THE TWR WAS TALKING TO THE ACFT ON FINAL AND DIRECTING THE ACFT BEHIND US TO POS AND HOLD. UNSURE OF MY POS AND KNOWING I WAS ESSENTIALLY ON OR BLOCKING AN ACTIVE RWY AND UNABLE TO COMMUNICATE WITH THE TWR I ELECTED TO TAXI CLR BY WAY OF THE TXWY JUST TO MY R. I EXECUTED AN S-TURN TO MY R AND L. I MADE A XMISSION ON TWR FREQ THAT I WAS TAXIING CLR OF THE RWY. TWR DIRECTED THE ACFT ON FINAL TO GAR AND PROMPTLY ASKED US IF WE WOULD LIKE TO RETURN TO RWY 22L FOR TKOF. I STATED THAT WOULD BE FINE FOR US AND WE WERE DIRECTED ONTO RWY 22L FOR BACK TAXI FROM TXWY S4 TO THE DEP END. ASKED IF WE NEEDED ANY DELAY ON THE RWY; WE RESPONDED NO; AND WERE CLRED FOR TKOF. THE ACFT THAT HAD BEEN BEHIND US HAD NOT YET TAXIED INTO POS AND WE DEPARTED IN FRONT OF IT. CONTRIBUTING TO THE CONFUSION IN THIS SIT IS THE FACT THAT THE AREA INVOLVED IS A LARGE SINGLE EXPANSE OF PAVEMENT. JUDGEMENT OF LOCATION AND DISTANCE FROM THE 2 INTERSECTING RWYS IS DIFFICULT. THE POSSIBILITY OF USING THE WRONG RWY IS VERY REAL. THE ONLY THING TO DO IS NOT MOVE UNLESS CERTAIN OF YOUR POS.