[Mithalwen]

We know that Boromir was a man of action rather than a scholar unlike his father and brothers and also that there was a relatively short time between the eve of the assault on Osgiliath on 20th of June when Faramir first had the "seek for the sword that was broken" dream and Boromir leaving on the 4th July and from what we know of the family dynamic it is a fair bet that the dream was only taken seriously once Boromir had had it. There was possibly not the time to seach the archives even if the knowledge existed there such things area often learnt by experience. From Elfhelm's words at the edge of the Grey Woods it seems that not that many of the Rohirrim knew the way unguided even to Minas Tirith, chief city of their closest allies ... true Gondorians were more likely to study maps and the like but knowing something exists is very different to knowing where it is... I know I own various pairs of scissors but can I find them when I need them?

I doubt that the location of Rivendell was exactly known other than to the elves and we know that the men of Gondor (and Rohan) had no dealings with them at that time. Did people in Dale know of Rivendell? The dwarves would certainly but a rather circuitous route.

Look at what Boromir says"the way was full of doubt and danger..long I have wandered by roads forgotten, seeking the house of Elrond, of which many had heard but few knew where it lay"

In the "Journeys of Frodo" the admirably thorough Barbar Strachey, says that it isn't clear what route he took though he claims 1200 miles. She says it is possible he took the road the whole way but I think it unlikely he went to Bree for reasons stated.

Outside Moria he says "Let us journey south to the Gap of Rohan...taking the road that I followed on my way hither" . But it is ambiguous whether by that he means the road he took to or from the Gap.

While Elrond says he arrives in the "grey morning", Hammond and Scull point out that the tale of years has him arrive the night before - which might allow him to be connected to Elladan and Elrohir arriving out of the wilds unexpectedly. Maybe they were the ones who "brought him in".