Don't...go alone. It's ok to run alone, and many people prefer this. But once you finish, not having anyone to share the experience with can take away from the excitement.
Do...ask someone to join you. Whether you have a friend or family member who is willing to run with you or someone waits for you at the finish line, the experience is much more enjoyable when your loved ones are present. Plus, who else is going to take dozens of photos of you at no charge?