What Zwift workouts would you recommend for building base fitness during the off-season, assuming a structured 12-week training plan with 3-4 rides per week, and how would you incorporate strength training and high-intensity interval workouts into the plan to maximize overall fitness gains? Would you focus on specific routes or workouts, such as the Tempo Ride or Endurance Ride, or would you create a custom workout plan using Zwifts workout builder? How would you balance the need for low-intensity, high-volume rides with the need for high-intensity interval workouts to improve anaerobic capacity and muscular endurance? Are there any specific Zwift features, such as the Workout of the Week or Training Plans, that you would utilize to add structure and variety to the training plan?