As with other post-run pains, pay attention to your fueling and hydration before, during and after your workouts. Your fatigue may be easily fixed with a change to your diet.
If you're working out on an irregular sleep schedule or not getting enough sleep to begin with, you may feel fatigued. Aim for a fixed amount of sleep every day, and stay consistent.
Have you ever fainted at a finish line? When the body works out, the heart pumps to push blood to the working muscles. When the body stops exercising, the heart slows down the pumping of blood even though the blood vessels are still dilated. Sometimes this means the body's blood pressure plummets, and the person can feel dizzy or faint.
To keep from feeling lightheaded after your run, cool down adequately to gradually reduce your heart rate and blood circulation. Don't panic if you feel dizzy after a run. Panicking will keep your heart rate from slowing to a normal rate.
Another reason for feeling lightheaded is a drop in blood sugar or a need for more calories. Try snacking on gels or chews during your run: Aim for about 100 calories of a carb-rich snack every 20 to 30 minutes, and then again right after you finish working out.
If your symptoms linger after trying some of these tips, visit your doctor for a blood test. The test may point out vitamin deficiencies that can easily be supplemented and get you back to training.