SMA CLRED TO L RWY; CAN'T SEE AN 'L' ON RWY ASSUMES PARALLEL TAXIWAY TO BE L RWY; LANDS.

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THE PROBLEM AROSE WHEN I RECEIVED CLRNC TO LAND FOR MY FIRST TIME ON 35L AT COS; 4 MI BEHIND A MLG WITH 4 PEOPLE ABOARD; SOME LUGGAGE; HALF FUEL; 6172 FT FIELD ELEVATION AND 85 DEG TEMP. MY ARPT DIRECTORY SHOWS ONLY RWYS 35 AND 30. NEW 35R IS APPROX 8500 FT E; OUT OF SIGHT WHEN ON FINAL TO 35L. I WAS UNABLE TO SEE AN 'L' ON 35. I ASSUMED THE BLACKTOP TO THE L OF 35 WAS 35L. MY CONCENTRATION WAS SOMEWHAT DIVIDED IN MAINTAINING DISTANCE BEHIND THE MLG TO AVOID WAKE TURB. BY THE TIME I REALIZED THE BLACKTOP WAS A TAXIWAY; I DECIDED IT WAS NOT SAFE TO TRY TO 'STEP- OVER' TO THE ADJACENT RWY BECAUSE OF FIELD ELEVATION; THE ACFT LOAD; THE ACFT HEIGHT AGL AND THE CONFIGN OF FULL FLAPS. A 'GAR' ALSO SEEMED UNSAFE TO ME. THE SAFEST COURSE SEEMED TO BE TO LAND STRAIGHT AHEAD AND STOP AS QUICKLY AS POSSIBLE. THE SAFEST COURSE SEEMED TO BE TO LAND STRAIGHT AHEAD AND STOP AS QUICKLY AS POSSIBLE. THE MLG HAD BY THEN TURNED ONTO THE TAXIWAY BUT WAS AT LEAST 3/4 OF A MI AWAY AND A STOP WAS MADE WITH PLENTY OF SEPARATION. TWR THEN ASKED ME TO CALL THEM BY TELEPHONE WHEN PARKED. I DID SO. ONE OF MY PAX ASKED A CAPT AND FO (AIRLINE UNKNOWN) IF THIS LAYOUT WAS CONFUSING TO THEM. THE CAPT SAID IT WASN'T AND THE FO SAID IT WAS CONFUSING TO HIM. PERSONNEL AT THE FBO SAID I WAS NOT THE FIRST TO BE CONFUSED BY THIS LAYOUT; RESULTING IN LNDG ON THE TAXIWAY.