That we are walking in the spirit means that we are acting and thinking about spiritual things. Those who walk in the spirit seek God above everything else, the book of Psalms tells us about the spiritual thirst, the thirst for God: "As the deer pants for streams of water, so my soul pants for you, my God. My soul thirsts for God, for the living God. When can I go and meet with God?" Psalm 42:1-2.  This teaching means that we must live and seek the perfections of God (the spiritual gifts are perfections): "Be perfect, therefore, as your heavenly Father is perfect" Matthew 5:48, "Be merciful, just as your Father is merciful" Luke 6:36.
Seeking God, or seeking spiritual things is not something that is simply desirable or beautiful, our actions are subject to spiritual laws that are eternal, such as the law of the sowing and reaping: "Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life" Galatians 6:7-8.
Jesus explains to us in the parable of the talents (Matthew 25:14-30), that the talents that the master gives to his servants are the spiritual gifts that God gives us, and those talents or gifts must be invested and give a profit, in other words, the meaning of the life is living and increasing the spiritual gifts; the spiritual gifts are progressive, expansive, edifying. We all have a ranking of righteousness, mercy, perseverance, and this ranking or score gives us prizes such as eternal life, the fruits of the spirit, and salvation. 
Beatitude constitutes us, surrounds us, and orders the universe towards harmony, human beings are not simply dust that returns to dust, life has a progressive meaning; walking in the spirit also means understanding that beatitude has a transcendent meaning for men. God has a plan for humanity and it is up to men to discover this plan through the search for spiritual realities.