Plain and simple the “secretary spread” is a combination of lack of physical activity (loss of muscle, and muscle tone) and an increase in body fat %.
You can do exercises that target the hips, butt, and thighs to increase muscle size and/or muscle tone. However there is no such thing as spot reducing fat content so, the key is finding a balance with increasing your physical activity to shape your butt and legs and lowering your calorie consumption to get rid of excess fat content or to maintain a healthy weight.
Here are some exercises to help shape and tone the Butt, Hips and Thighs. http://bit.ly/vlutH1
and as always check with your doctor first to make sure that these exercises are ok for you.