> used to solve hundreds and hundreds of classical mechanics problems for fun, in his spare time. [emphasis added]

You kind of missed your own (or possibly your professor's) point there: being great at something is a matter (in part) not just of putting in the work, but of having the inclination and motivation to put in so much work. If you think of it as "all the toil that gets you to that level", you're not the kind of person who is going to become great in that field.