What specific workout protocols and periodized training plans are most effective for targeting and improving muscular endurance and cardiovascular adaptations necessary for success in long-distance gravel endurance races, particularly those lasting 6-12 hours or more, where the demands of sustained effort and repeated energy expenditure over varied terrain are extreme, and how do you prioritize and balance volume, intensity, and recovery within a structured training block to optimize performance and minimize the risk of injury or burnout, considering individual rider characteristics and the highly variable nature of gravel racing?