Star Wars fans have had to twist themselves into pretzels to make sense of Han's statement. The prevailing (theory?) is that the Kessel Space Run goes through a series of black holes, so that flying straight through involves certain death. A daring captain will cut corners on the route to make it shorter by going close to the event horizons. So, flying the Kessel Run in under 12 parsecs is actually a super difficult feat.

Or Lucas just didn't know.