I didn't explain clearly enough. Spiritually, the only thing we can do to make ourselves better is surrender to Christ and have the attitude "not my will but your will be done." We do this through Bible Study, finding out how we should live and act from scripture and applying it to our lives. We can choose to follow this advice from scripture and become more like Christ, or we can choose to do the opposite and be drawn further away from Christ, spiritual atrophy so to speak. A big part of becoming more spiritually fit is realizing that we must surrender to the power of the Holy Spirit. It is He who works in our lives, molding us in the image of Christ. If you have accepted Jesus Christ as your Savior, then the God who made the universe lives within you and desires a relationship with you.
I strive to live my life as an act of worship to my God. Spiritually, I read the Bible, pray, surrender my will to His, and show others the love of God. Physically, I strive to worship God through how I take care of the temple that is my physical body, watching what I put into it, exercising it to keep it healthy and strong, keeping my mind sharp through study, etc...
Sorry for the long confusing answer. Hopefully it helps explain my previous answer. In a nutshell, we can only make Spiritual progress through the power and control of the Holy Spirit. We can't do anything of our own strength to draw closer to Christ, and to true happiness which is a life of devoted worship to our Creator, following His will.
Have a great weekend