One of the key features of training program design is the manipulation of stimulating and maintenance workouts. The general pattern I typically use is to introduce one new workout per week (A), habituate to that stress, then, in a subsequent mesocycle, add a second workout of similar stress per week (2As).
After habituating to that stress (2As), replace one of those workouts with a new workout of a different stress (1A, 1B). Habituate to that stress, then, in a subsequent mesocycle, take out the older stress (A) and replace it with a second workout of the newer stress (2Bs).
After habituating to that stress (2Bs), replace one of those workouts with a new workout of a different stress (1B, 1C). Habituate to that stress, then, in a subsequent mesocycle, take out the older stress (B) and replace it with a second workout of the newer stress (2Cs).
Repeat this pattern as you progress throughout the training program. For example, add one acidosis threshold workout per week, then a second, then remove one and add a VO2max workout, then add a second VO2max workout per week, then remove one and add an anaerobic capacity workout, then add a second anaerobic capacity workout per week.