Agreed, when there's a blood sucking villain coming after me, I shall skidaddle rather sharply myself too. But Martha had just met this wierd guy, he's gots two hearts (which she oddly smiled at), and she decides to grab his hand and run away down a corridor with him.
I think it was too soon to have them doing that, because the two lead characters holding hands and running away from the baddies together is quite a symbolic image. It's one of partnership and trust, which I don't think they should have after meeting each other that soon. Especially when you consider that the Doctor lost his love in Rose, shouldn't he be a little distant?