Here are three biblical reasons for exercise from David Parrish on The Resurgence:
1. Paul wrote to the church in Corinth that our body is God's temple where his Spirit dwells (1 Cor 6:19)
2. Our bodies are members of Christ and should not be abused (1 Cor 6:15)
3. Our body is not our own, it is God's. How dare we desecrate it on purpose? (1 Cor 6:19-20)
What a lot of people forget is that exercise can be simply walking for 30 minutes a day while chatting with a friend, playing with their dog or taking a post-dinner bike ride. The important thing is to get moving as a starting point.
The flip side of needing more exercise is where I can easily fall if I'm not careful, making exercise an idol that I hold in a clenched fist, unwilling to let go if I can't do it when I want to and how often I want to. My heart can so quickly fall into a legalistic idolatry with exercise where I set rules for myself that must be satisfied or I become upset or guilty. This idolatry is something that I must continually war against, remembering that my body is Christ's temple, not for my own good, but for the glory of the Father. There are times when I will have to sacrifice my own wants so that I can worship God with my time, my energy, my relationships with others, my heart. At these moments, I have to remember 1 Timothy 4:8. "Physical training is of some value, but godliness has value for all things, holding promise for both the present life and the life to come."