Clearly, she just likes to play. And, remember, she was gone for 10! years. So, though she's still 42, she doesn't have the wear-and-tear as if she had played for those 10 years.
I love that all her old friends from the tour say she's crazy -- "Kimiko, WTF?" Lol. She's been back since 2009. I wonder how many more years she'll play.