The commentaries explain that there is a great difference between the ages of those before the flood and those afterwards. That is a manifestation of the enormous climatic changes that God wrought in the world to deliberately make the world a less hospitable place. The Sages teach that one of the most central problems of the generation of the flood was the ease that they lived their lives. Their physical sustenance was to a large extent taken care of, they had no real worries and that is what led them to behave in the most base and degenerate fashion. Though God had punished Adam and mankind now had to toil to grow food, the amount of work necessary to was minimal in comparison to the amount of work necessary after the flood.
