As one commentator has put it, fleeing would mean "selecting a slow death over a quick one. The death toll would be very little affected by any plausible relocation effort, since Earth's ability to support life would be universally diminished."
The amount of soot and floating ash from the impact and following fires would blot out the sun, certainly for months, possibly for years, disrupting growing cycles. In 2001 researchers at the California Institute of Technology analyzed helium isotopes from sediments left from the later KT impact and concluded that it affected Earth's climate for about ten thousand years.
This was actually used as evidence to support the notion that the extinction of dinosaurs was swift and emphatic—and so it was in geological terms. We can only guess how well, or whether, humanity would cope with such an event.
And in all likelihood, remember, this would come without warning, out of a clear sky.
But let's assume we did see the object coming. What would we do? Everyone assumes we would send up a nuclear warhead and blast it to smithereens. The idea has some problems, however. First, as John S. Lewis notes, our missiles are not designed for space work. They haven't the oomph to escape Earth's gravity and, even if they did, there are no mechanisms to guide them across tens of millions of miles of space.