Eating well is not easy in a culture that surrounds you with glazed donuts, McNuggsts, sugary drinks, and a plethora of other disease-inducing food-like products. A commonly misunderstood 'cure' for leading a lifestyle that supports poor eating habits is solely hitting the gym and burning off calories. Exercise is important, but the equation is not so simple. It is true that having a calorie deficit will ensure weight loss, but this is not to say that you will be in good health.
According to Dr. Laskowski at Mayo Clinic, the average person should exercise at least 30 minutes per day (210 minutes per week). More active individuals may increase the time spent working out to 300 minutes per week, depending on the person's workout or weight loss goals. For someone who is following a strictly balanced intake of food (whole foods such as fruits, vegetables, whole grains, legumes, and plenty of water) this is a recipe for great fitness. However, for someone who adopts a lifestyle that includes a large portion of processed foods, sugary drinks, and other such 'junk' foods, going to the gym does not automatically reverse that poor diet.
This chart created by Women's Health (based off of information provided by the USDA National Nutrient Database) shows you briefly how long you would have to spend working out to burn off the calories that are in milkshakes, sodas, cheeseburgers and fries, and doughnuts. Not only would one need to spend an outrageous amount of time working off those empty calories, but the body would not be getting the proper vitamins and minerals needed to maintain healthy functionality. An individual could hypothetically burn off all of the calories obtained through 'junk' food in a day, but this is where a line is drawn between weight loss and overall wellness.