With little recent experience at the airport; a light aircraft pilot crossed Runway 30 at BZN without a clearance.

Narrative

Apparently I was just familiar enough with BZN airport to be dangerous. I've been to BZN a few times in the last 15 years -- the last time 3 years ago. I had always taxied on the parallel Alpha to one end or the other; and departed. Today I had landed; refueled and picked up my passenger (my brother; who two days ago had told me he had advanced cancer). I noted that they were still landing/taking off on the same runway; but the distant windsock showed a crosswind and perhaps favoring the other end. So when Ground gave the wind and asked if I wanted to use Runway 12; my reaction; as I accepted; was 'oh; they are changing ends'. They also said something about me having a choice of back taxiing at Alpha; which I didn't understand; but thought it would become clear as I reached that end of the runway. So I taxied out onto Alpha; heading for the opposite end of the runway from which I'd landed. After taxiing a distance; Ground called 'you've just crossed 21'. My reaction was to jam the brakes; prompting urging in no uncertain terms to not stop; to clear the runway. I was told later that an airplane was on short final as I crossed near the 1/3 point of the short runway; but I have no first-hand knowledge. In retrospect I'm amazed at the errors I made without a thought; and I don't know what to blame except 'familiarity' which apparently bred complacency; exacerbated by the same-sounding Runways 21 and 12. For future prevention I plan to challenge myself to be suspicious of my knowledge of the airport; and to review the various runways which exist.