This is exactly what Cal Newport talks about in So Good They Can't Ignore You. He calls it "deliberate practice" and gives the examples of playing guitar. He (

This is exactly what Cal Newport talks about in So Good They Can't Ignore You. He calls it "deliberate practice" and gives the examples of playing guitar. He (Cal) started playing around the same time as [professional guitarist] and claims to have likely played around the same number of hours. The difference was that he stuck to songs he was comfortable with. If he ever tried to learn a song that was out of his reach, he would give up and learn a different song. Contrast with the other guy, who would constantly push himself to not only play songs just outside of his comfort zone, but push himself to play them faster. I believe he used a phrase that really stuck with me: the idea of "being comfortable with being uncomfortable".