Of course, you should start by talking to your doctor. Rule out any serious medical reasons for low energy. Make sure your heart and lungs are OK, that your iron levels are good, that everything is where it should be. Then start to think about diet and exercise.
You know that, if you need to, you can get moving, and keep moving. You can walk around and around the mall for a couple of hours, Christmas shopping with your grandchildren. And the more often you get moving like that, the easier it will be. If you add some kind of activity choice to your routine 3 times a week, you'll get your heart, lungs, and muscles in better shape, and feel like you can burn that energy.
The other side of the equation is taking in the energy you need. If you aren't losing weight, it would seem you are taking in enough calories. If you've got body fat, you've got calories to burn. So why don't you feel like you have the energy to burn those calories? Calories ARE the energy, aren't they?...
Sometimes, though, the body does not seem so eager to "spend" calories. Sometimes it would rather save them. Everybody's body is different, but, in general, a high level of insulin in the blood will cause the body to start trying to store available calories as fat. Blood sugar is actually shunted off to where its calories can be used to produce fat, ready to be stored in your fat cells.
High levels of insulin have been linked to diabetes, heart disease, weight gain, high blood pressure, and low energy. Too much insulin can cause temporary hypoglycemia (low blood sugar), which is accompanied by brain "fog", fatigue, temporary weakness, irritability, and headaches.
Insulin is not bad. It is necessary. The body releases it when blood sugar levels go up. It alerts the muscles and organs to take up the sugars they need for energy. But if you ring that "insulin alarm" hard, every day, the muscles start to ignore it. That's called insulin resistance. If the muscles don't take up the blood sugar, then the liver is going to use it to store fat in your fat cells. If you REALLY push things hard, then even the fat cells and liver stop listening to the insulin alarm after a while. Then you will be stuck with too much sugar in your blood. That's a form of diabetes.
If you don't actually have diabetes, you "only" have to worry about artery disease, heart disease, high blood pressure, bad cholesterol, high triglycerides, strokes, and hypoglycemia.
You can read about the effects of insulin resistance in popular diet books like the South Beach diet, the Carbohydrate addict's diet, and many other low-carb or "good-carb" diets. What all these diets have in common is a way to combat the insulin problem by dealing with the blood sugar problem.
Remember -- generally -- insulin is produced when your blood sugar goes up. When you frequently spend time in the "high sugar" state, your body gets tired of you ringing the "insulin alarm". If this is your problem, simple changes to your diet and exercise might improve blood sugar levels and reduce insulin resistance. You won't know until you try.