I believe that fitness and healthy eating is an important part of life. We are only given one body in our life and we need to take care of it. God wants us to be strong and ready to work for his kingdom.
"She clothes herself with strength, and strengthens her arms" (Prov. 31:17).
"What? Do you not know that your body is the temple of the Holy Spirit, who is in you, whom you have received from God, and that you are not your own?" (1 Cor. 6:19).
"Now no discipline seems to be joyful at the time, but grievous. Yet afterward it yields the peaceful fruit of righteousness in those who have been trained by it. Therefore lift up your tired hands, and strengthen your weak knees" (Heb. 12:11-12).