Stuff that I like

There are some hints that the distance to Rivendell varies depending on who you are. Frodo starts approaching the Ford in late afternoon; he is in desperate need of healing, and is brought to Rivendell midway into that same night.

In The Hobbit, in contrast, the dwarves and Bilbo cross the Ford of Bruinen in the morning, and the sun is down by the time they reach Rivendell. There’s lot of references to the journey being longer than Bilbo would expect:

They came on unexpected valleys, narrow with steep sides, that opened suddenly at their feet, and then looked down surprised to see trees below them and running water at the bottom. There were gullies that they could almost leap over, but very deep with waterfalls in them. There were dark ravines that one could neither jump over or climb into. There were bogs, some of them green pleasant places to look at, with flowers growing bright and tall; but a pony that walked there with a pack on its back would never have come out again. It was indeed a mich wider land from the ford to the mountains than you would ever have guessed. Bilbo was astonished.

Then there’s Aragorn’s line when Merry asks him how far it is to Rivendell:

“I don’t know if the Road has ever been measured in miles beyond The Forsaken Inn, a day’s journey east of Bree. Some say it is far, and others say otherwise. It is a strange road, and folk are glad to meet their journey’s end, whether the time is long or short. But I know how long it would take me on my own feet, with fair weather and no ill fortune: twelve days from here to the Ford of Bruinen.”

(By the way, it always amazes me, now I’ve noticed it, that the hobbits manage this journey - which Aragorn says would take him 12 days on the Road, with “fair weather and no ill fortune,” in only 14 days with Frodo severely injured, travelling mainly off the Road, and with some bad weather and wrong directions. Some of that’s due to the extremely fast pace Glorfindel sets for the last twoand a half days, but it’s incredibly impressive.)

One thing it makes me wonder about though is Boromir, and particularly that the book mentions he arrived in Rivendell "in the grey morning", which personally I've always taken to mean in the very early morning, when there's some light but the sun may not yet be properly up and the dim light has that kind of greyish quality (granted that it's late November, so even with Rivendell being further south in Middle-Earth than my home in the real world, at that point the sun probably rises quite late so it may not mean as much as otherwise). Idk maybe that's just my reading of the text, but given that earlier in the chapter the morning is described as sunny, so the "grey" can't refer to cloudy weather, that's how the phrasing would seem to me, anyway.

And it's always struck me as a little bit odd in the sense that, for that to be the case, he would've had to either both start his journey very early that day (which, granted, in itself is plausible) and have been very close to Rivendell already when he started, or he would have had to push through the night without stopping to rest at all. And to me it's kind of, on one hand if he knew he was close enough to Rivendell to reach it that early, wouldn't he have known he was close, and rather pushed through in hopes of getting somewhere inside before the night was over, instead of spending it camped by the roadside in cold damp late November weather? But if he was so far from Rivendell as to have to travel the whole night through without stopping to reach it in early morning, then how would he have known he was close enough for continuing through the night, instead of stopping to make camp and sleep, to be worth it for reaching Rivendell faster?

But your point about the distance to Rivendell being different to different people in different times would make either of those more plausible, either being shorter than he thought to allow him to reach Rivendell in time to attend the Council (since Elrond implies that those who were in the Council had been called there by fate or some such power and were meant to all be there on that day to play their part in the events at hand), or being longer than he thought it would and forcing him to travel all night to reach it (since he was a stranger, unknown to Elrond and coming unexpectedly).