Arya is the Last Hero, and Sandor is her/his dog. The whole thing is about Arya, with Sandor.   First bold is referencing Nymeria, obvious Arya connection and a parallel. It also puts a time frame to things. Eventually the narrow sea will run dry and Arya will lead the realm of men (or convince them) into the dried out seabed, as Nymeria lead her people to the sea to eventual salvation. What she's doing, she's doing on Bran's orders. His whispers. She's going for a magical purpose