Spiritual fitness is an interesting topic for many of the same reasons. We think that if a person looks right, if they do the right things at the right time, that they are spiritually fit. In the spiritual realm, it would be impossible for someone to be spiritually fit without Christian fellowship, but being in church on Sunday does not make a person spiritually fit. As Christians we try to equate how we appear with how 'spiritual' we are. But the same truths hold. To be spiritually fit I need to: properly fuel my spirit and exercise my spirit as it needs.
To feed my spirit, I need to be in God's Word. I cannot control how much my spirit grows through feeding, but I can control the amount and quality of the food I give it. I could read 20 chapters skimming and not get much food. I could read 2 verses flippantly and not get much food. But I could read any amount of scripture prayerfully and looking for understanding and benefit my spirit greatly.
To exercise my spirit, I need to live out what God's Word tells me. Firstly, I need to exercise myself unto Godliness. Secondly, I need to take advantage of ministry opportunities that God brings into my life. I have the opportunity to disciple my wife and children, the opportunity to give to my church, the opportunity to pray for the needs of others, the opportunity to give to missions, the opportunities to meet needs in the lives of others through my time, labor, or finances. We could list dozens of opportunities that each of us have everyday. Our spiritual fitness depends on proper feeding, and how many of these opportunities we take. This is not to equate business with fitness, and we certainly might find it difficult to meet all the needs before us, but as we stretch to care for the properly prioritized opportunities in our lives, we give ourselves more opportunities to become more fit in our spirits.
"For bodily exercise profiteth little: but godliness is profitable unto all things, having promise of the life that now is, and of that which is to come."
- I Timothy 4:8