It is stressed here a couple times that the way to Rivendell was not precisely clear. Gandalf, who had been there before and was generally considered well-travelled in this part of the world, was only able to get them there with some difficulty. The implication is that maps and/or navigation are not so well established as they are now, and perhaps not as well as they were in the days of the Numenorean navigators.
The elves of Rivendell seem to know who Bilbo is, and Bilbo is curious as to how this came to be (and never finds out, as far as we are aware). This means that either Gandalf just told them (he had met a few Rivendell elves just prior to the encounter with trolls), or else they knew somehow before. Did they perhaps know something of his ancestors, the Tooks? On the other hand, we have evidence that Bilbo might have been meeting (or trying to meet) elves previously. JRRT wrote a snippet later in which Gandalf, speaking to his hobbit companions and Gimli the dwarf after LotR, recalls how he came to choose Bilbo for his expedition against Smaug. He relates that Bilbo was not at home when he first went to find him, and his neighbor said that he was off hiking, and had mentioned a hope that he might see elves. In Fellowship of the Ring we discover that companies of elves sometimes travel through the woods of the Shire, normally unseen by hobbits; perhaps they had seen Bilbo before, even if he never saw them up to that point.
Chapter 4