What are the most critical factors to consider when designing a gran fondo training plan for a rider with limited experience in long-distance events, and how can a structured approach to training be balanced with the need for flexibility and adaptability in order to avoid burnout and prevent overtraining, while also ensuring that the rider is adequately prepared for the physical and mental demands of a gran fondo, and what role do periodization, specificity, and progressive overload play in the development of a effective training plan, and how can a rider with limited experience in structured training best incorporate these principles into their training regimen in order to maximize their chances of success in a gran fondo, and what are some common mistakes that riders make when designing their first gran fondo training plan, and how can these mistakes be avoided in order to ensure a successful and enjoyable experience.