Commercial instrument pilot reports mistaking SUU for APC and landing without clearance in ground fog.

Narrative

[I] departed IFR (VMC) from CCR enroute [to] STS; [landed at STS] and departed 1.5 hours later VFR (VMC) to return to CCR. [The] ATIS [at] CCR reported IFR; ground fog; visibility .25 miles; [and cloud] tops at 300 FT. Obtained IFR clearance and made two LDA [Runway] 19 approaches; both of which were missed with no runway visual contact. Diverted to APC and canceled IFR under VMC. Controller; upon request; provided vectors and runway was observed ahead. Reported field in sight and was told to go to 1200 [squawk] code. I went to APC frequency; [the] Tower had closed; and landed. [The] runway was clear from above but; once within 50 FT; the visibility was .2 miles. Slow taxied to exit to [the] taxiway and proceeded to enclosed ramp area. [A] security truck approached [and I] was directed to shut down and get out; which I did. [I was] taken into custody and informed that I was at SUU and not APC.[My mistake was] focusing on landing on the in-sight runway. [I] should have activated portable GPS to ascertain APC distance as well as tuning in APC VOR for direction and DME distance. [I] was premature in canceling IFR with Controller and should have remained on IFR with Controller until at APC. [The] Controller did all that he was required to do; but an alert to me before turning me loose on VFR with the distance to APC would have probably alerted me that the runway ahead was not APC; which was another 20 miles ahead.