God has given us minds – minds which need constant renewal and growth in line with the gospel (Rom. 12:2) so that we see our world and ourselves in line with truth. Using them well is an act worshipful obedience (Rom.12:1), and using them badly is an act of idolatrous rebellion (Rom. 1:18-32).
Staying fit and healthy is an option that most of us can make choices about, and if they increase the chances not just of living longer, being married longer, seeing our great-grandchildren, but of being useful in ministry for longer, I reckon most of us would make an effort if we thought about it.