use it or lose it

“If you don’t use it, you lose it,” the ex-marine said as he leaned against the side of the pool and after two hours of laps up and down its length.
It was senior swim day. His skin held to his 80-year-old body firmly. His squinting eyes looked as though they were bordered with the tattooed lines of a sun dial’s second hand.
His wife passed last week. He walked beside her as her body slowly faded.
“No matter how old your body gets,” he said. “Your mind still imagines you’re 20.”