I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. Mind not high things, but condescend to men of low estate. Be not wise in your own conceits. In most of Paul's writing he divided up his letters, half with doctrine and half with duty except in the book of Romans.
Romans leans heavily on doctrine.
In Romans 12 Paul speaks about our duties of Christians. We have had some duties laid before us and we need to be serious about these duties. It is time for us to get serious with our relationship with God. Is says in verse 1 that we first need to give our body as a living sacrifice to God. It is not our body, it belongs to God.