It took less than a second for Martha to take off. Running is something she's good at. Running from things trying to kill her? Even better. Only there's no Doctor here to help her and she's trying to remember where she put that phone thing.
She took off at a 90 degree angle, hoping direction change would slow the creature down. She had to get somewhere safe. And none of the buildings currently around her looked as though they'd be any help. The library. Had to get to the library.