I don't remember the episode title, but the runway was built back in season 3. Remember Sawyer and Kate breaking stones and Sawyer spending more time looking at Kate than working, and therefore invoking the wrath of Picket? That was the runway. In a later episode Juliet joked with Sawyer that it was a runway for aliens.
"I handed in the new RJ novel with the
working title, BY THE SWORD. David says the sales force loved the title at the
pre Turkey-Day sales meeting, so that's what it will be. That means Paul Ramplin
gets a credit line in the acknowledgments."