Does walking on a treadmill help you lose weight and slim your legs?

Any amount of cardio exercise will help reduce all-over body fat. The higher intensity the cardio, the more calories you burn, the more fat you lose. You should also enlist a light strength training workout to tone up and build your muscles. Don't worry about your muscles getting too big -- we're not men, our bodies do not build the same way they do. Don't worry about losing weight, just toning up. According to your hight to weight ratio, you are at a healthy weight. Good job :)