Don’t underestimate your capacity to get fitter by repeating the same training. When you complete a training cycle, you are not the same athlete you were at the beginning. Hence, if you start your next build before you’ve lost much of the fitness you gained, the same training will affect you differently. In fact, your zones are NOT your zones (unless you’re talking about heart rate). Fitter means faster, so what I’m really saying is that you should be able to do the workouts in the early weeks of your next cycle faster than you did the first time, which means they aren’t really the same workouts, and on you’ll go from there. If the training still seems ridiculously easy, then okay, go ahead and pad some of the workouts with an extra interval or an extra 15 minutes, but again, don’t underestimate your capacity to get fitter by repeating the same training.