2.5 miles, actually.
I love Chelsea bashing, but - speaking as someone who used to live on Union Square West - she wouldn't have had any trouble making it down there in the time frame she claims. It was roughly 1 hour 20 minutes from the first plane impact until the South Tower fell. It only takes about 15 minutes to walk a mile.
According to Chelsea's breathless account, she didn't leave Union Square until after the second tower was hit. She then tried to call her mother, and then left Union Square. That time frame gives her approximately 45 minutes of walking downtown before the South Tower fell. Under normal circumstances, she may have been able to travel 15 minutes in a mile, but with so many people fleeing uptown at the same time, I seriously doubt she made it too far in that time frame.