Whats the optimal time frame for a self-trained cyclist to consider transitioning to a coach-led program, and at what point does the cost and commitment of working with a coach outweigh the benefits of continued self-directed training, especially for those who have already achieved a certain level of success and fitness through solo training, and are the benefits of a coach-led program more significant for athletes with a specific goal in mind, such as completing a first century ride or competing in a local race, or can even casual, non-competitive cyclists benefit from the structure and accountability of a coach-led program, and how do you determine whether a coach-led program is right for you, and what are the key differences between working with a coach who specializes in a specific discipline, such as track or mountain biking, versus a more generalist coach who can provide a broader range of training and guidance.