The section of the rules in which the rules of forced walking appear, entitled "Speed", reads as follows:
Each character and monster has a speed that corresponds to the distance in feet that the character or monster can travel in 1 round. This number implies short, energetic movements in a situation where life is in danger. The following rules determine the distance traveled by a character or monster in a minute, hour, or day.
As you can see, the movement rate rules are designed to calculate distances for longer-term continuous trips. Generally, in a dungeon, the characters do not move continuously at a steady pace, as the pace rules assume. Instead, the characters usually stop and explore, and often fight for short periods.
In addition, the rules of "forced march" specify:
Forced walk. The Travel Pace chart assumes that characters travel for 8 hours a day. They can push beyond this limit, at the risk of exhaustion.
For each additional hour of travel beyond 8 hours, the characters cover the distance shown in the Hour column, and each character must make a Constitution save roll at the end of the hour. The CD is 10 + 1 for each hour after 8 hours. On a failed save throw, a character suffers from a level of exhaustion.
As you can see, they are even more explicitly defined to cover trips of more than 8 hours a day. Generally, if a "dungeon" requires more than 8 hours of continuous travel to cross it, it's not so much a dungeon as an extensive cave system probably connected to the Underdark.