A C172 PLT DEPARTED A GRASS STRIP VFR; ENCOUNTERED IMC; AND FILED AN IFR FLT PLAN AIRBORNE WITH CENTER FOR AN ILS INTO ZZZ.

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I PLANNED TO FLY A C172/G FROM ZZZ TO ZZZ2. FOR THE PURPOSE OF A PRE-BUY INSPECTION; THE ACFT HAD BEEN PREVIOUSLY REPOSITIONED TO A SMALL GRASS STRIP 7 MI S OF ZZZ. THIS ARPT IS IDENTED AS ZZZ3. I NEEDED TO GET THE ACFT FROM ZZZ3 BACK TO ZZZ FOR THE PURPOSE OF FILING AN IFR FLT PLAN AND TO PURCHASE FUEL. MY INITIAL PLAN WAS TO MAKE THE 5 MIN FLT TO ZZZ UNDER VFR; AND THEN FROM THERE I WOULD FILE IFR TO MAKE THE 1394 NM TRIP WITH INTERMEDIATE FUEL STOPS. I DEPARTED VFR FROM ZZZ3 AND TUNED IN THE ATIS. I LEARNED THAT ZZZ WAS CONDUCTING IFR OPS. (MUCH TO MY SURPRISE!) THEY WERE RPTING A 900 FT OVCST CEILING AND 2 MI VISIBILITY. I CONTACTED THE TWR AND TOLD THEM THAT I WANTED TO LAND VISUALLY. THE TWR RESPONDED THAT THEY WERE UNDER IFR AND HANDED ME OFF TO CENTER. THERE WAS SOME REAL TENTATIVENESS IN MY VOICE WHEN THE CTLR HEARD ME SPEAK AND THIS MAY HAVE ALARMED THEM. I WAS THEN GIVEN AN IFR CLRNC AND A SQUAWK CODE. I REQUESTED AN ILS TO ZZZ AND WAS GIVEN RADAR VECTORS FOR THE ILS APCH. I WAS THEN HANDED BACK OFF TO THE TWR; FLEW THE ILS APCH AND LANDED. THE TWR PROCEEDED TO QUERY ME (MY NAME; HOME BASE; AND WHETHER OR NOT IF I WAS INST RATED). I RESPONDED THAT I WAS AN INST RATED; COMMERCIAL PLT. THE CTLR CHIDED ME FOR FLYING 1 MI INSIDE OF THEIR AIRSPACE WITHOUT A CLRNC. I SIMPLY APOLOGIZED TO HIM. THERE WAS NO FURTHER INQUIRY FROM THE TWR. THE PROB AROSE FROM THE FACT THAT I WAS IN A HURRY TO GET THE ACFT BACK TO AN ADEQUATE FACILITY FOR FUELING AND FLT PLANNING. INITIALLY; I HAD NEGLECTED TO FILE AN IFR FLT PLAN ON THE GND PRIOR TO DEP. I FIGURED THAT SINCE IT WAS SUCH A SHORT FLT; IT DID NOT REQUIRE SUCH INTENSIVE FLT PLANNING. MY MINDSET WAS; 'I CAN GET IN UNDER VFR AND LAND VISUALLY.' THIS ILLUSION WAS QUICKLY SHATTERED AND THE REALITY OF BEING IN 'UNEXPECTED IMC' SET IN. IT CAUGHT ME COMPLETELY OFF GUARD WHICH CONTRIBUTED TO MY UNEASE. IT WAS THE FIRST TIME THAT I ACTUALLY FELT 'PRESSURED' AS A COMMERCIAL PLT. I CORRECTED THE SIT BY PROMPTLY CONTACTING THE APPROPRIATE ATC FACILITY AND CONFIGURING THE AIRPLANE FOR THE APCH. AFTER ALL; I AM A COMPETENT INST PLT. IN THE FUTURE; I WILL BE SURE TO PERFORM MORE THOROUGH FLT PLANNING. ALSO; AS A STANDARD OPERATING PRACTICE; I WILL PREPARE AN IFR PLAN (EVEN IN VMC) SO THAT I AM NEVER CAUGHT IN THIS PREDICAMENT AGAIN. EVERY FLT; EVEN A SHORT 7 NM JOURNEY; REQUIRES THE UTMOST PROFESSIONALISM; SAFETY; AND COMPETENCE.