Private pilot reported a runway and taxiway incursion was due to confusing taxiway markings and signage at intersection Zulu/Alpha at STS Airport.

Narrative

Sold an aircraft a year ago; I had flown it many years from ZZZ; uncontrolled; avionics original; steam; used hand held mic. Renting Aircraft X with headset and PTT and Garmin 430. About 10 hrs renting. Generally when taxing to Runway 32 told take Zulu/Alpha to runway. This time told taxi 32 via Alpha. On Alpha and when I got to Zulu was confused for a moment and started down Zulu; I find arrows on signage at Zulu/Alpha intersection unclear; crossed limit line that is about 25 yards from runway; thought cleared to cross to get to runup. Knew not cleared onto runway and would not have entered without clearance; saw Aircraft Y on about 2 mile final. I was never a threat to Aircraft Y. Trying to confirm I had made correct turn; realized I hadn't and turned to go back to continue correctly. Ground called my N number after I was already turning back and I replied 'sorry I had made a wrong turn.' Proceeded to runup and take off without incident. Told later they told Aircraft Y to go around; if I was threat to Aircraft Y why didn't they tell me to stop as I went down Zulu?