how did the hound "not touch the world"?

Of course, Montag is thinking metaphorically at this point -- the hound does not literally move around without touching the earth. What I think that Montag or the narrator means here is that the Hound is a sort of a symbol of the society as a whole.

I think that what is pursuing Montag at this point is not so much the physical Hound as it is the weight of the society as a whole. Montag feels the pressure of the society coming after him (again metaphorically) as he prepares to renounce the society forever. He has lived his whole life in this society and so he naturally feels this pressure as he rejects the society.