This past week, I saw part 1 of a PBS show on Lewis & Clark. I believe it's a repeat, so many of you may have seen it already earlier in the year. Aside from the unusually large number of laxatives they took on the expedition, I was mainly interested in the inspirational aspects. It's a well-documented lengthy expedition under conditions that weren't medieval, but were still low-tech enough to be a good guideline for pre-19th century wilderness travel.
Almost the entire journey is by river. Upriver, actually, so it's not as quick as you'd expect river travel to be, although it is quicker than it would have been overland. Because of the great distance involved, it took a couple years to complete. Part of the time was spent in "winter phases", when the Corps of Discovery didn't travel at all because that would have been foolhardy. A few days here and there during the rest of the journey were spent visiting/negotiating with inhabitants along the way. Lots of time was lost going around a couple obstacles.
The expedition had a general map of most of the territory, but they mapped their actual journey in detail: every little bend in the river, every landmark visible along the way. I think we tend to treat mapping in RPGs a bit cavalierly, assuming that any maps found are completely accurate at any scale. Players tend to skip mapping if they already have a map of the area. Eventually, the expedition reached unmapped territory... and later, discovered that even their general map of the continent was wrong, and the Pacific Ocean was not as close to the Rocky Mountains as they expected.
The expedition heard stories about a "monster" they'd never encountered before: grizzlies. When they first met a grizzly, it was pretty fearsome, but they beat it and felt pretty cocky. Several more encounters, though, and they began to wish they'd never seen one: sure, they were winning their monster encounters, but these things were taking way too much time to kill, compared to ordinary bears. And they didn't seem to get scared away as easily as other bears.
There were a lot of other things about the expedition that would make good examples of things a band of adventures might have to deal with during a long river journey. Overhanging trees wrecked the mast of their boat. The safe part of the river in many places was close to the banks, but the banks were soft and sandy, tending to collapse suddenly into the river and nearly capsizing them. They came to unexpected forks in the river and had to guess which one actually came from the mountains. Waterfalls forced them to lose a month of time as they carried their boats overland, around the obstacle.
But the main thing all this made me think of was: boy, I'd like to see more very long journeys, instead of short journeys of a couple weeks, with most of the details glossed over.