Typical outlines of Paul say he is still dealing with or discussing justification in Romans 5.  Then he “turns” to sanctification in Romans 6.

But Romans 6 is simply an elaboration and application of what Paul says at the beginning of Romans 5.  “More than that, we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us.”  In Romans 6 we find these sufferings have to do with the death and resurrection of Christ giving us new obedence, and then the Role of the Spirit is again visited in Romans 8.

Paul is not building successive stories in a structure, but circling around and revisiting the same concepts over and over in order to help us grow in our understanding.

And though Romans 5.12ff presupposes federal headship, and thus can be used to prove imputation (much better than Romans 4.5, for what it is worth), Paul is obviously not trying to prove such or arguing for it.  He’s arguing for what we now call Postmillennialism.  He is promising that the glory and salvation to come now that Christ has died and risen will far exceed the horrors of sin and death.

That is what Paul says.  Just as we “rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us,” so we rejoice that

Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned— for sin indeed was in the world before the law was given, but sin is not counted where there is no law. Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.

But the free gift is not like the trespass. For if many died through one man’s trespass, much more have the grace of God and the free gift by the grace of that one man Jesus Christ abounded for many. And the free gift is not like the result of that one man’s sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification. For if, because of one man’s trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.

Therefore, as one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men. For as by the one man’s disobedience the many were made sinners, so by the one man’s obedience the many will be made righteous. Now the law came in to increase the trespass, but where sin increased, grace abounded all the more, so that, as sin reigned in death, grace also might reign through righteousness leading to eternal life through Jesus Christ our Lord.

A last point: it does not follow from the fact that Paul believes in both justification and sanctification, and that he distinguishes them, that he must only deal with them in stages in a letter which are exclusively devoted to one or the other.