A Real Basket Case by Beth Groundwater.
Groundwater's first mystery novel stars Claire Hanover, 40-something with too much time on her hands, despite her burgeoning gift-basket business. Claire's husband, Roger is obsessed with progressing in his business, and spends little time with her.
She decides to take a fitness class, thinking that she (and Roger) might feel better about herself if she's in better shape. Enrique, the handsome instructor flirts with her, and offers to give her private training. Hesitant at first, her friends convince her to do it.
The next thing Claire knows, Enrique is dead in her bedroom, and she's being interviewed by the police.
Claire is a strong character (other reviews have called her "gutsy" and "plucky"). To me, she's just a woman who sees what needs to be done and does it. A hard book to put down, I stayed up waaaay too late so I could finish it.