The U.S. Army's physical fitness testing involves push-ups, sit-ups, and a two-mile run. Resistance training (lifting weights) can improve your ability to do all of these things. To begin, you may want to get supervised assistance at a local gym. You'll benefit by gradually incorporating push-ups and sit-ups into your routine. In addition to weight lifting, walk briskly every day, gradually increasing your walking speed. Then turn your  walk into a slow jog. Try to increase your pace and distance by small amounts each week. By being patient and consistent, you'll improve your performance while reducing your risk of injury.