When the run is only days away you should hopefully be physically ready, however Sports Psychologists agree to optimize performance, runners must also prepare mentally.
Be ready to experience some discomfort as this is normal with prolonged exertion. The key is to acknowledge these feelings rather than just hope the pain or fatigue you feel will just go away. Disconnect from them and shift your focus to other things. By reframing these physical sensations you change the emotional response to the feelings.
Studies have shown that the hardest part of any distance event is the middle section and I would therefore recommend that you strategize to conserve both energy and concentration for this period.
Finally, goal setting is a powerful tool for improving performance; simply by setting yourself a target time to complete your run and visualizing achieving it, can help you strive to accomplish just that!