A few general points: To me, " make oneself fit", though not actually wrong I suppose, is less natural than " keep fit" (if you're already fit) or " get fit" (if you're not). "Keep one's body fit", though again not wrong, is also less natural to me than just "keep (oneself) fit" (unless I particularly wanted to emphasise physical fitness over, say, mental fitness). I would personally more naturally say "they/the girls exercise " or "they/the girls do exercise s ", rather than "they/the girls do exercise".
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