Build and maintain your muscles, use it or lose it or however you want to say it. Your muscles will shrink (atrophy) if you don't continue to do strength training (muscle does not ever change into fat). And, the body fat will come back with a vengeance! Fat cells will shrink but they will fill up again when you are inactive for a long period of time.
If you are not exercising, you could be hit with a double dose of likely bad news. First, you will probably gain weight (interpreted fat). Also, your metabolism begins to slow down in your thirties. Your metabolism will slow down even more because you are also losing muscle mass. Muscle mass speeds up your metabolism because your body has to work harder to maintain it.
In your early thirties, you can begin to lose as much as one-half pound of muscle each year. Inactivity will make this problem even worse. So, you begin to lose muscle mass and gain body fat
Fortunately, muscle mass can be maintained/increased (and body fat decreased) throughout your life with regular weight training (includes bodyweight exercises). Weight training also helps to keep your bones strong (preventing osteoporosis).
Eat enough to build muscle and burn fat. You do this by following your meal plan based on your basal metabolic rate and level of daily activity.