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By improving your eating habits, you will decrease your body fat and fatigue, while gaining muscle and endurance. Not eating right could be what is holding you back from breaking your 800m record.  Eat a variety of wholesome foods like fruits, vegetables, whole grains, and lean meats. This will improve your body composition. Increase protein and the right amount of carbohydrates, as these will provide your body with the nutrients to build muscle and to give your body energy.  If you’re training for 30-45 minutes, shoot for about 3 grams (0.11 oz) of carbs on training days. If you’re training for 46-60 minutes, shoot for about 5 grams (0.18 oz) of carbs on training days.   Avoid processed high-sugar and high-sodium foods. These will hinder your body from running efficiently. Eat the right pre and post-workout carbohydrate-rich snacks. These should be consumed an hour before working out and within 30 minutes after a workout. Bananas, protein shakes and non-caffeinated energy bars are ideal. Your body needs about two liters on an average day to stay properly hydrated.On days when running, you need to drink extra water to compensate for your output and prevent yourself from slowing down. Don’t just chug extra water though; listen to your body and drink when thirsty.  Pre and post-workout hydration is vital. An hour before you start your run, try to drink about 16 ounces (450 g) of water. Drink the same amount of water after running. Look for signs of dehydration. Feeling thirsty or experiencing dry mouth, headaches, light-headedness, lack of urination and constipation are all the body's ways of letting you know that you are dehydrated. If you begin to experience any of these symptoms, it's best to increase your water intake. By stretching properly, you can improve your stride and your stride length (increasing your speed). Stretching before and after your workout will also help to prevent injury.  Muscles to stretch include your leg muscles (hamstrings, quadriceps and adductors) and your lower body (hip flexors and glutes). Try to add two or three longer stretch routines a week to your workout schedule.  Do some yoga to help you with your flexibility. You won’t be able to run a faster 800m in one day. Just like training for a marathon, the best way to achieve your goals is to set a schedule. By following a training plan you can perform at a peak performance, even when running a shorter distance.  Set an end date and work backward. Even with an 800m run, pick a date that you want to accomplish this run with your best time ever. Do one or two quality and intense workouts a week. These include running hills or tempo runs. Pick rest days where you can do some other sort of moderate exercise like yoga to stay limber. Ensure that you get enough sleep before the race.
Eat the right diet. Keep properly hydrated. Stretch properly. Set a training schedule.