zk,
The only question that matters is: "What are your symptoms?" Isolated LAE is of absolutely no concern. Furthermore, there is nothing that could be done about it. Even more, if you were to stop exercising because of LAE, you would be doing something detrimental to correct something that is of no consequence.
The increased size is almost certainly from athletic heart, but the finding should prompt you to ensure that you do not have undiagnosed high blood pressure.
Don't let LAE affect your life, for peet's sake.
Good luck.