Answer:
She can make her own decisions. You are her parent, though, and you can always give advice. :) Perhaps just say to her, "I've noticed that since you quit walking so much you are gaining weight. Shall you and I schedule some time to walk together? Maybe 3 or 4 times a week?" Then set up the times and days. It would be good for both of you, exercise and catch-up time.
Your child is an adult and as such must make her own decisions on such matters. All you can do is encourage her to knock off the weight and don't worry about offending her. Sometimes it is the only way to make your point. You can't fail, only she can. If you have the money to do it you might join a health club and add her too then suggest you work out together several times a week. Once she develops a habit of exercise she will not want to quit.