Fitness, bike handling, and peloton etiquette. Far too many people are unaware of how to corner in a group, what not to do like change direction or brake suddenly, all the skills us "old lags" learned with a club. Try going out with a club first, get the idea of riding close and safe, and learn the skills, which will help in your early races. One golden rule, never, ever, ride close to anyone who is/has recently been a triathlete. Most (sadly) have the bike handling skills and knowledge of a plank.
Good luck...