The travel speeds given in the Travel Pace table assume relativelysimple terrain: roads, open plains, or clear dungeon corridors. But adventurers often face dense forests, deep swamps, rubble-filled ruins, steep mountains, and ice-covered ground---all considered difficult terrain.
You move at half speed in difficult terrain---moving 1 foot in difficult terrain costs 2 feet of speed---so you can cover only half the normal distance in a minute, an hour, or a day.