The capitol wasteland is in much worse shape than the Mohave. From what we see in Fallout 3, people generally eat squirrels, iguanas and pre-war food.
The capitol wasteland pretty much works on the basis of scavenging and trading with the occasional herd of brahmin or small farm here and there.
Fallout 3 also has a big emphasis on water, Megaton has a water purifier that is about to give out while other settlements just drink irradiated water or trade with merchants.
The main story in Fallout 3 is terrible but the world building is pretty solid overall.