You need to lift weights. Young, old, male, female, lifting heavy things is an important key to being healthy.
Lifting weights doesn't mean going to the gym and doing bench presses. It could be push ups or pulls ups at home. Can't do 10 push ups? Do 10 negatives (where you start in the plank/top position and let yourself down. Then you can put your knee down, get back up, and do it again).
I know some women are worried about doing weights because they 'don't want to look like that'.
Here's what women look like if they lift weights: http://www.facebook.com/GirlsGoneStrong
Look at the pictures. Those are not muscle bound body builder type women. (Yeah, some of them do, but look at Nia Shanks an Molly Galbraith). That's what weight lifting can do you for you.
Warning - link is to facebook and has some before and after pictures of women in bikinis.