Hey all : I am out for the day - I see Jen posted the Green Dragon for me already! - Thanks Jen!
That is a question I had in the Two Towers... when the Rangers and the sons of Elrond showed up in Rohan, how did they know that Aragorn would even be there? Maybe... through Galadriel since she knew the general direction the Fellowship was heading, but when Aragorn left Lorien he hadn't decided whether he would go with Boromir to Minas Tirith or with Frodo to Mordor... But, then, plans went astray when they were attacked and Merry and Pippin were carried off. There is no way that Gandalf rode Shadowfax all the way back to Rivendell and back. It took Boromir 110 days to get from Gondor to Rivendell.
That's a question that just stuck with me... maybe those who have read the books more than me know the answer.