If your mind is like mine (and I hope it isn't), the news about the gym will quickly cause you to think about her motivation for getting in shape and making her body look better. From there, the imagination just goes everywhere, none of it good . . .
It [censored]. All you can do is try to acknowledge the thoughts, not try to fight or control the feelings, and find healthy distractions.
Me: 46 W: 44 Married: 17 Together 21 D13; S10 BD: 03.03.15 (Not attracted to you) Almost 2 years trying, alone, to save marriage Status now: Divorced (effective 06.13.17)