If you're convinced that God wants you to take better care of yourself, don't make obedience too complicated. Don't wait for someone to give you a compelling reason to do something you know in your heart you should be doing. Don't hope to find a way to exercise that excludes hardship and sacrifice. Don't wait to "feel good" about your decision before committing yourself. If you know what God wants you to do -- even if it's something as simple as walking a couple of times a week -- obedience isn't complicated. Stop hoping it will be easy. The Lord has given you the opportunity to grow in your faith and obedience. He wants you to trust his grace and rest in his unfailing strength.
As I reflect on the 6 decades I've been a follower of Christ, I can't think of a single sermon encouraging me to take care of my body. Yet as teachers of God's word, it is both our privilege and responsibility to do so. Everything within us recoils from making big changes in our lives. Nevertheless, I absolutely believe that the Lord can give you a desire to obey him in this area of your life if you'll ask him for help.
This is perhaps a good time to pause and seek the Lord. I invite you to pray, "Lord, guide me into your will in this area of my life. Help me to step out in faith in you regardless of what change is necessary. I want to obey you, Lord, when it comes to taking responsibility for my health, and I don't want to live outside your will."
Then be prepared for him to strengthen your faith muscle so that it will become stronger and stronger with use.