It is possible to burn 1000 calories a day, but it requires a high level of physical activity and may not be sustainable for long periods of time. To burn 1000 calories in a day, you could do activities such as running at a 10-minute mile pace for approximately 2 hours, swimming laps for about 2.5 hours, or playing tennis for 4 hours. Other ways to burn 1000 calories include circuit training, biking at a moderate to vigorous pace, or even walking briskly up hills for 7.5 hours. It is important to note that these are all estimates and that actual calorie expenditure may vary depending on individual factors such as weight, body composition, and intensity of activity.