Learning a piece of classical music is not easy, and can be draining. But you get past the learning stage, and get to the point where you’re no longer learning, just playing and perfecting and improving. When you get to that point, the piece no longer saps your energy. It gives it back, and increases it. What a comfort to know that no matter how much energy a piece takes from you now, it will give it all back and then some in the long run.