It was a daunting doubleheader.

The conductor Yannick Nézet-Séguin, the next music director of the Metropolitan Opera, had spent the day leading a dress rehearsal of Richard Strauss’s dense “Elektra,” which opens on Thursday. And it wasn’t long before he was due back in the pit to conduct the evening performance of Wagner’s nearly six-hour, stamina-testing “Parsifal.”

As he walked back to his hotel near Central Park during his brief break last Friday, Mr. Nézet-Séguin considered fitting in a nap. Instead, he opted for a workout — using a smartphone app to connect with a personal trainer — and a light supper.

“It’s very simple moves,” he said of his exercising afterward, as he walked back to the opera house. “Mostly stretching, just to make sure that tonight is good.”