UNAUTH RWY ENTRY RESULTED IN CLOSE PROX WITH ACFT ON LNDG ROLL.

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I WAS TAXIING FOR A TEST HOP AFTER MAINT. EXTENSIVE CONSTRUCTION HAS BEEN IN PROGRESS AT THIS AIRPORT FOR ABOUT 60 DAYS WITH NUMEROUS TAXIWAYS; IR AND 19L CLOSED AND 14/32 CLOSED MOST OF THIS 60 DA PERIOD. INITIAL TAXI CLRNC WAS TAXI ACROSS 1R TO 32. I REQUESTED 1L BECAUSE OF XWIND AND ROUGH SURFACE CONDITIONS OF 32. RECLRNC WAS TAXI TO 1L-HOLD SHORT OF 32. WE WERE PERFORMING TAXI CHECKS AND BEFORE I REALIZED IT WE WERE JUST PAST THE EDGE OF RWY 32. I STOPPED ABRUPTLY JUST AS A SINGLE-ENG SMA PASSED IN FRONT OF US ON HIS LANDING ROLL-OUT; NO EVASIVE ACTION WAS REQUIRED BY EITHER ACFT. I WAS STUNNED TO SAY THE LEAST AS TO HOW I GOT TO THAT POSITION WITHOUT REALIZING IT. I WAS OCCUPIED WITH TAXI CHECKS AND THOUGHTS ABOUT SYSTEM CHECKS THAT WE WOULD BE DOING DURING THE TEST FLIGHT. I'M FAMILIAR WITH THE AIRPORT HAVING BEEN BASED THERE FOR 10 YRS--SO THAT WAS NOT A FACTOR. THE FACT THAT I HAD BEEN TAXIING ACROSS THAT CLOSED RUNWAY UNRESTRICTED FOR THE LAST 60 DAYS WAS A FACTOR. UPON RETURNING FROM OUR TEST FLIGHT WE CROSSED THAT SAME INTERSECTION AND WHAT I BELIEVE TO BE THE PRIMARY CAUSE BECAME EVIDENT. THE HOLD-SHORT LINE WAS 60-75% OBSCURED BY PAVEMENT REPAIRS AND THE REMAINING HOLD LINE WAS BADLY WEATHERED MAKING IT DIFFICULT TO SEE. I'VE BEEN FLYING PROFESSIONALLY FOR OVER 20 YRS AND I HAVE AN INGRAINED HABIT OF CHECKING FOR TFC ON ALL RUNWAYS ACTIVE OR NOT UPON XING THE HOLD SHORT LINE. I BELIEVE THAT HAD THE RUNWAY BEEN PROPERLY MARKED THIS INCIDENT WOULD NOT HAVE HAPPENED.