It's ok to lust. Even Jimmy Carter had "lust in his heart" and he's been a good husband to Rosalyn all these years.
If your soulmate is secure about herself -- and of course, she IS secure about herself or you wouldn't have chosen her as your soulmate -- she won't mind if your eyes are wandering while you master the Stairmaster.
Just don't ruin a nice dinner (or anything else) with her by going on and on about the anatomical feature(s) of your lust objects. Keep that to yourself.
Oh yeah, one more thing. As a public health professional, I fully endorse exercise for physical and emotional health. Therefore, I must advise you that if lusting is keeping you from getting a good work out, the lusting must stop.