I know what you mean. It feels like a two day trek from Africa to Everest to Dinosaur, with nary a diversion in sight.
Still, the area of DAK that you walk in is hardly bigger than the other parks. I think it is all more psychological. The effect of vast distances is created by all that empty space. Well, empty...filled with that fifty foot tall bamboo.
The distances would be more manageable if there was something interesting to see or hear or walk in. But disappointingly, for a park that sells itself as the exploration park, there is actually very little to explore in many areas.
However that may be, Avatar is not going to change any of that. It's going in Camp Minnie Mickey - more far flung corners of the park to walk to!