This isn’t really spelled out in the books so I’m documenting it here.
Terrain Costs for Overland Movement are multiplied to the distance of the terrain.
For example, travel over 5 miles of rocky Desert terrain, with a terrain cost of 2, becomes effectively 10 miles of travel.
The PHB states that the players can move their inches rate in miles in "a half day's travel". Would that imply that a full day's travel is double that number? One would assume that means 12 hours, but if you cross-reference those numbers with the DMG, you'll notice those numbers are still too low. But they do work out if you use 14-16 hours of travel per day.
AD&D 1e assumes 16 hours of travel per day.
It may not be the most realistic metric, but I like the idea that characters are spending the entire day sans rest time, traveling. The 12 hour travel rule otherwise gives characters 4 hours of downtime in which they are not sleeping or traveling. AD&D 2e explicitly states 10 hours of travel, allowing for 6 hours of downtime and 8 hours of sleep every day. Kind of a waste, I think, even if it is more realistic.
