What specific types of Zwift races are most effective for building endurance, and how do the varying formats, such as hilly routes versus flat courses, impact the overall training benefit? Are longer events, such as 50-60km group rides, more beneficial for endurance training than shorter, more intense 15-20km time trials, or do the benefits lie in incorporating a mix of both styles into a training plan? How important is the role of pacing and strategy in Zwift races for building endurance, and are there any particular tactics that can be employed to maximize the training benefits? Are there any specific Zwift events that are geared towards endurance training, and if so, what sets them apart from other types of events? What metrics or data points should riders focus on to measure progress and optimize their endurance training within Zwift?