Athletes in certain endurance sports are at risk of certain types of overuse injuries, but these injuries are not the only problems that can develop as a result of a heavy training schedule. Over-training is itself a common problem for people in endurance sports, but it's not always one that's easy to recognize.
Cycling is all about efficiency. It is easy to find stuff we can buy for our bikes that are supposed to make them faster, but the benefits of an efficient training schedule far outweigh any gains that a new set of wheels or carbon bars can provide. A well thought out training plan involves the 2 main components required to improve performance: Overload and Recovery.