According to Paul, we are living in a completely new age. No longer is the law the only way to righteousness, nor was it ever intended to be a way back into God. However, this is a righteousness which has existed since before all time. It is like electricity. It has always existed, but only recently have we been aware of it's power and existence. Indeed, if it had not existed since the beginning, how could the law and the prophets bear witness to it? Rather, this has been the plan of God all along, to send us His son Jesus, the lamb of God slain from the foundation of the world.
God, before even making the world, counted every single cost and plotted every single detail. He knew what would be involved in making such a world as ours and he made it anyway. He knew of our sin and our shame, about Eden and the inevitable fall, and the death of His Son crushed on the cross, and he accounted for all of it by giving us His righteousness and salvation so that we might not perish. God always has a plan. With a God who gives so much attention to detail, who could worry?