What are the most effective Zwift races for speed work, and how do their specific characteristics - such as route profile, length, and competition level - contribute to their suitability for interval training and increasing power output, considering factors like short punchy climbs, longer steady-state efforts, and sprints? Which Zwift races offer the best opportunities to practice different types of speed work, such as VO2max intervals, lactate threshold work, and neuromuscular training? Are there any Zwift races that are particularly well-suited for simulating real-world racing scenarios, allowing riders to practice tactics and pacing strategies in addition to working on their speed?