basically, because of the nearby blackhole, in order for the distance of the Kessel run to be made shorter ("less than 12 parsecs"), you need a ship that is fast enough that it can travel closer to the blackhole without being sucked into oblivion....SO, in order to make the Kessel Run in less than 12 parsecs, Han would need a seriously fast ship ("able to make .5 past light speed").
HOWEVER, I think ric is probably right about how AT THE TIME, GL was just making stuff up to sound cool. But once the Star Wars universe took off, they constructed a story to go around the "less than 12 parsecs" thing, so it would make sense with the definition of "parsecs" that came later.