As a recent high school graduate, I realize that sports are over for me. But that doesn't mean that I shouldn't still stay in shape. I have to continue to workout in order to maintain my physical fitness.
The same is also true for our spiritual lives. We must continue to workout our faith, even after we leave church and are no longer in fellowship with believers. We must exercise spiritual fruits in order to gain spiritual strength.
It can be easy for us to maintain fitness while we are involved in sports, but it takes extra discipline and self-control to workout in the off-season. Likewise, when we are involved in a ministry at church, or in the company of other believers, it can be easy to produce spiritual fruit. But, when we are at work or school or even home, the battle is stronger and discipline is crucial.
As followers of Christ, we must be devoted to maintaining spiritual fitness and health. And just like we use equipment when we exercise, God has given us equipment for spiritual exercise: the sword of the Spirit, the belt of truth, breastplate of righteousness, the gospel of peace, shield of faith, the helmet of salvation and prayer. (See Ephesians 6.) We should use each and every piece of equipment in order to gain strength and sustain fitness.