AN SMA CROSSED AN ACTIVE RWY WITHOUT CLRNC. IT HAD BEEN TOLD TO HOLD SHORT.

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WHILE ACTING AS PIC; I CROSSED AN ACTIVE RWY WITHOUT ATC AUTHORIZATION. THIS OCCURRED AFTER LNDG ROLLOUT. DURING THE LATTER PART OF OUR RETURN LNDG ROLLOUT; ATC INSTRUCTED ME TO HOLD SHORT OF RWY 28L BEFORE XING. WE HAD JUST LANDED ON RWY 28R. I ACKNOWLEDGED ATC'S HOLD INSTRUCTION. THE INSTRUCTOR (CFI) THEN STATED 'TURN L HERE OR WE WON'T MAKE IT' (TO THE TIEDOWN AREA). I BEGAN A L TURN ONTO WHAT I THOUGHT WAS THE TAXIWAY. I WAS ACTUALLY TURNING ONTO THE DIAGONALLY INTERSECTING RWY 23. AHEAD; ON THE L SIDE OF RWY 23 LAY A DISABLED SMA. IN RECOLLECTION; AT THAT TIME; I SEEM TO HAVE BEEN PREOCCUPIED WITH LOOKING FOR THE HOLD SHORT LINES ON THE SURFACE. (ERRONEOUSLY SINCE WE WERE ON RWY 23; NOT A TAXIWAY). AS WE TURNED TO AVOID THE SMA; IT BECAME STARTLINGLY APPARENT THAT WE WERE IN THE INTXN OF RWYS 28L AND 23. ANOTHER SMA WAS ON SHORT FINAL FOR 28L. DISTANCE ESTIMATED 2000 FT. I SAID TO MY INSTRUCTOR THAT WE WERE 'ON THE L.' HE THEN APPLIED A SHORT BURST OF PWR TO ACCELERATE THE ACFT THROUGH THE INTXN. ATC THEN ASKED IF I HAD PREVIOUSLY ACKNOWLEDGED THEIR HOLD SHORT INSTRUCTION. I RESPONDED THAT THAT WAS AFFIRMATIVE. ATC TOLD ME TO PHONE THE TWR ON RETURNING. IT WAS EXPLAINED THAT THE INCIDENT HAD TO BE RPTED.