Scripture says, "The wicked run when no one is chasing them." So you are excused if you skipped your morning run.
But seriously, why should Christians take care of their bodies? It's simple. The essence of Christianity is to love and serve others. Being healthy is a way to care for those around you. Especially as you get older, you will need to have the strength to help your children and grandchildren. You will also be less of a burden by helping to make your aging easier on them. Plus, when they see your health and vitality, they will want to imitate that when they grow old.
Someone says, "But Jesus didn't exercise." That's only technically true. As for cardio: Jesus and his disciples walked from town to town. As for muscles: They did physical manual labor without any power tools. As for hiking: they had hills to climb as well as stairs to ascend (people often slept on rooftops). As for weightlifting: they had fishnets to haul in. Makes me tired just thinking about it. In my opinion, Jesus and his disciples would have been in great shape. They didn't need to worry about going to the gym or on a run. Lifestyle then required physical exertion on a daily basis. Plus there was no junk food. Everything was organic and homemade. We are living in a vastly different culture.
Augustine said, "Take care of your body as if you were going to live forever, and take care of your soul as if you were going to die tomorrow." Being healthy lifts up everyone around you. I can hardly think of a more Christian act.