While the term "in shape" is relative, you'll probably notice your fitness level increase within a few weeks of starting a well-planned exercise routine, according to MayoClinic.com. How long it takes to reach your fitness goals, however, depends on what shape you're in when you begin. Any improvements can make you stronger and boost physical endurance, and the key to success is sticking with your exercise program for life.
If getting in shape means reaching a healthy weight, consider the numbers: It takes a deficit of about 3,500 calories to lose 1 pound of body fat, which you can achieve in a week by burning 500 more calories than you eat per day. To burn roughly 500 calories through exercise, a 155-pound person could spend an hour using a stationary bike, rowing, race walking or in-line skating. You need at least a day or two of rest from exercise each week, however, so realistically you'd lose 1 pound every eight or nine days with these activities if you don't reduce calories at the same time.
Measuring cardiovascular fitness is one of the best ways to judge how in shape you are. Using American Heart Association guidelines, a person with a healthy cardiovascular system has a total cholesterol level of less than 200 milligrams per deciliter, has blood pressure readings below 120/80 and has a fasting blood-sugar level of less than 100 milligrams per deciliter. While only a health professional can tell you where you stand and how long it may take to reach healthy levels, the AHA recommends ensuring cardiovascular health with at least 150 minutes per week of moderate aerobic exercises such as brisk walking and casual cycling, or 75 minutes per week of vigorous cardio such as running and speed cycling.
Strong muscles are another indicator of being in shape, and resistance exercises increase your strength in a matter of weeks. Such exercises include lifting weights, using resistance bands or using your own body weight for resistance with pushups, crunches and lunges. To shape up your whole body, perform resistance exercises for each major muscle group two to three times per week: legs, buttocks, abdomen, back, chest and arms. For optimal strength training, choose a weight or resistance tough enough that you can barely perform 12 repetitions. Once you can perform 15 reps with relative ease, increase the difficulty.
Rather than trying to get in shape as quickly as you can, start with a manageable routine that suits your comfort level. For example, walk for 30 to 60 minutes per day and gradually ramp up to running. Begin each heavy exercise session with a five-minute warmup of light to moderate cardio, and stretch only after warming up or at the end of your workout -- stretching cold muscles can lead to injury. If you're inactive now, see your physician before embarking on a fitness routine.