Thanks Bret and Green....but this was a single base. SO the runner has to see where the fielder isset up and pick her path accordingly?
She is always allowed the 3 ft' running lane in foul territory. It is only if she is out of that lane and hit with the ball that the ump can rule interferece.
But as MTR correctly points out if the 1B is set up on the foul side of 1st to receive a throw she can go into fair territory and should not be called for inerference even if hit with the throw.