All of which is very stupid, because a parsec is a measure of distance, not time. :-)

I've read a few explanations. One of which from Lucas is that basically Solo was blowing smoke. However, the explanation from the books detailing his life prior to meeting Luke, et al was that the Kessel Run was a dangerous route to travel and that by running it in a short *distance* he had flown a faster, but much more dangerous route; proving both his skills and the ships abilities