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The DMG's figures are based off the physics of the D&D world. Walking in D&D is the same as taking a single move action each round and no other actions, meaning an unencumbered human can walk 30 ft in 6 seconds. D&D uses a 6000 ft mile (I crunched the numbers to figure this out), so it takes a human 20 minutes to walk one mile. This a human with a 30 ft movement speed covers 3 miles an hour, or 24 miles in one day. It wouldn't work in the real world, but in D&D it works fine.