get in(to) shape
To get into good, robust health; to become strong or fit. Boy, I really need to get into shape. Ten years working behind a desk have given me quite a belly! Wow, Jim has really gotten in shape lately.
get into shape (or get someone into shape) become (or make someone) physically fitter by exercise.
get (yourself) (back) into ˈshape take exercise, etc. to become fit and healthy (again): After she had the baby, she started swimming every day, to get back into shape.