Page 264 of the Unfinished Tales remarks in passing that "the courage and hardihood required" of Boromir to make his journey from Minas Tirith to Rivendell "ws not fully recognized in the narrative". Boromir himself remarks (page 239 FotR) that it took 110 days and that it covered (page 365) 1200 miles. Are the following points worth discussion:
1. It seems odd that the heir to the Stewardship would leave without a riding horse, a pack animal or an escort. There is no mention that he has a bow and arrows (hence no hunting), a spear or any other weapons besides a sword and shield.
2. It is hard to imagine that he would have to walk 400 miles to Edoras without being detected by the Rohirrim. They do loan him a horse at some point. Presumably, had he been obliged to walk the whole way, it would take something like 3 weeks. I would like to think he could have bought (or been given) more rations in Edoras. Perhaps even an escort to the Gap of Rohan.
3. He manages to cross Dunland alone. It is difficult to believe that Saruman would not have been aware of this transit. Surely, Boromir would be a valuable hostage. Whether Boromir, an experienced military leader, would have noticed signs of Saruman arming for war is not easy to determine. Realistically, the only route north would be on or along the old road. One would think near-starving Dunlanders might seek to waylay solitary travelers.
4. Later, the Black Riders and Gandalf (separately) ride from The Fords or Isen to Sarn Ford in five days - about 100 miles per day. It seems reasonable to expect that Boromir could manage 50 miles per day on a normal horse, so another two weeks to Tharbad. There he loses his horse, which manages to return.
5. Page 240 of FotR says "few knew where it [Imladris] lay" - perhaps that means someone in Rohan gave Boromir some directions. Maybe these directions state that Boromir must cross northward at Tharbad to avoid the Swanfleet Marshes. If so, that leaves him the problem of crossing the Mitheithel (Hoarwell). He never mentions taking the Last Bridge. An alternative might be to follow the east bank of the Bruinen northeast. Either way, he has 75 days to go 300 miles. It is the case in Middle Earth that "not all who wander are lost", but it would be hard to describe Boromir as knowing where he was going.
6. And time for Boromir is critical - he will need to journey hundreds of miles back before Sauron really attacks. At the time of the Council of Elrond Boromir has already been away for four months. The Fellowship and Boromir do not leave Rivendell for two more months. A solution to the dream/riddle might well be irrelevant.
7. On page 45 of The Hobbit the dwarves, Gandalf and Bilbo cross the Bruinen at the Ford. It takes them most of the day to reach the actual valley of Rivendell (probaby 3 miles as the crebain flies, but a lot longer following the white stones). Whether Boromir was aware of the stones or found them 80 years later is not known. I note in passing that if Asfaloth knew the route it seems like he and Frodo should just have kept running. If Asfaloth had to search out the route, the Black Riders (were they actually able to cross the water) would have had a much easier time simply closing in on their quarry as the white horse would be easy to follow visually.
something he gets