The travel speeds given in the Travel Pace table assume relatively simple 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.