SMA DEPARTS WRONG RWY; FORCES ACR GAR AT ATL.

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MYSELF ALONG WITH ANOTHER PLT (PVT RATED) FILED AN IFR FLT PLAN TO MAI (HOME BASE) WX AT DEP FIELD WAS VFR. FIELD OF DEP WAS ATL. CLRNC TO TAXI WAS AS FOLLOWS 'TAXI TO RWY 8R; HOLD SHORT OF 8L TAXIWAY CHARLIE. AFTER HOLDING AT 8L; WE THEN PROCEEDED TO TAXI TO 8R VIA A SHORT TURN ON DELTA TAXI THEN R TO BRAVO. TAXI WAS AN EXTENDED ONE. DUE TO JUST PICKING UP A FRIEND WHO HAD FLOWN IN FROM HOLLAND; THERE WAS A MODERATE AMOUNT OF CONVERSATION. I WAS FOLLOWING PROGRESS OF TAXI ON ARPT DIAGRAM. TAXI SPD WAS SLOW DUE TO PVT PLT ON CTLS. COCKPIT LIGHTING WAS EXCESSIVELY DIM. I ASKED FOR A FLASHLIGHT TO USE WHICH WAS ALSO VERY DIM. UPON ARRIVING AT END OF RWYS 8L ON BRAVO TAXIWAY; ATL TWR ASKED US IF WE WERE 'READY TO GO.' I ANSWERED AFFIRMATIVE; WE WERE AT HOLD LINE OF 8L WHEN TWR CLRED US FOR TKOF. I VISUALIZED THE ACFT ON FINAL; BEING ATL IT IS NOT UNCOMMON TO SEE. LNDG LIGHTS OUT ON FINAL WHEN DEPARTING. WE THEN ROLLED OUT; LINED UP AND DEPARTED. I THINK THE REASON FOR DEPARTING ON 8L MAINLY BE ATTRIBUTED TO AN EPISODE OF MILD CONFUSION IN THE COCKPIT. UNFAMILIARITY OF ARPT. SIMILAR RWY NUMBERS. I THINK WE HAD R TURNS ON OUR BRAIN AND TURNED R ON 8L INSTEAD OF L TO 8R. MISTAKE WAS QUICKLY REALIZED WHEN ACR (HVY) QUESTIONED TWR AS TO WHY WE WERE ON RWY. I IMMEDIATELY STARTED TO TURN R AFTER TKOF AS CLRED TO 160 DEG TO AVOID CONFLICT. ACR WAS ORDERED TO GO AROUND; 'WHEN TWR REALIZED' WHAT HAD HAPPENED. TWR WAS OBVIOUSLY NOT SEEING WHAT WAS TAKING PLACE. I UNDERSTAND COMPLETELY WHAT HAPPENED AND REALIZED THAT MY SHORT EPISODE OF 'RECTAL-CRANIAL INVERSION' WAS THE MAJOR CONTRIBUTING FACTOR. I HAVE BEEN FLYING MANY YRS AND THIS IS THE FIRST TIME I HAVE COMMITTED SUCH A MISTAKE OF THIS MAGNITUDE. LUCKILY THERE WAS NO NEAR MISS OR CONFLICT CLOSE CALL AS COULD HAVE BEEN. I HAVE LEARNED MY LESSON TO KEEP MY MIND SOLELY ON TAXI DIRECTIONS; PROCS; ATC; ETC; INSTEAD OF 'TALKING WITH FRIENDS.' IT WAS A TERRIBLY EMBARRASSING SITUATION AFTER IT WAS OVER; AS I REALIZED WHAT THE ACR CREW MUST HAVE THOUGHT. I WILL SINCERELY STRIVE TO WORK HARD NOT TO MAKE SUCH A MISTAKE IN THE FUTURE.