So, I've come into distance running AFTER having been a strong hiker and a crummy runner for a long time. I think the ultrarunners need to spend a bit more time listening to backpackers, and bit less time listening to marathoners. Back in the 90s, the hiking community was never as smitten with the HYDRATE HYDRATE HYDRATE mentality as the runners were.
In my 20s, I belonged to a hiking club that routinely covered 16-20 miles in our day hikes, so outings were about as strenuous as possible without actually breaking into a run. There was another hiking club in the area that maxxed out at 6 mile hikes, and the OTHER club had the motto "Hydrate or DIE!!!!" Like that was even possible while hiking in the metro NYC area. Yes, hikers have died of dehydration before, but not on a day hike because it takes more than a day to DIE of dehydration. We used to rip on them so hard. We were like, "Oh, here comes the AMC! Does anyone have any spare water so nobody dies?!?!?" "Rock paper scissors for who is going to carry out the bodies this time - someone only brought 1.5L of water" We used to carry water, of course. (Recommended - 2 liters per person). But we all just drank when we were thirsty and usually had some left over at the end.
In my 20s, I belonged to a hiking club that routinely covered 16-20 miles in our day hikes, so outings were about as strenuous as possible without actually breaking into a run. There was another hiking club in the area that maxxed out at 6 mile hikes, and the OTHER club had the motto "Hydrate or DIE!!!!" Like that was even possible while hiking in the metro NYC area. Yes, hikers have died of dehydration before, but not on a day hike because it takes more than a day to DIE of dehydration. We used to rip on them so hard. We were like, "Oh, here comes the AMC! Does anyone have any spare water so nobody dies?!?!?" "Rock paper scissors for who is going to carry out the bodies this time - someone only brought 1.5L of water" We used to carry water, of course. (Recommended - 2 liters per person). But we all just drank when we were thirsty and usually had some left over at the end.