Whatever it is, it's behind her. She's nervous. And she doesn't have a perception filter to help her. She's wearing trainers, so she can run if she needs to. A DVD is not going to help her. Her eyes flit here and there for a person or a weapon. The library is straight ahead and around the corner. She'll make it. She's Martha Jones.
Just to be safe though, she picks up a wayward stick that hasn't been picked up yet apparently and quickens her pace. Not running. Martha doesn't want to. But her imagination is in full gear and she needs something to feel secure.