The Bravo series follows the bubbly blonde and brunette, who are also roommates, as they build their fitness brand. But as their business soars, the two struggle personally when Kat and her fiancé Brian make it clear it's time for them to move out.
"It's never a good time for any big move," Kat tells TVGuide.com. "We were in the middle of producing a DVD, we were in the middle of filming our first season of a national television show and then we were ... talking about moving out -- It didn't feel right. We had lived together for so long [so you'll see in the finale] that it stresses me. I don't want things to change. I don't do well with change!"
Also in the one-hour finale, Kat says, "You get a sense of the direction that our company is going in, and the community and what to expect for Season 2 if that were to happen." Adds Karena, "Tone It Up is growing up and they're going to get married and maybe they'll have babies!"
Watch a sneak peek of the Season 1 finale, which airs on Thursday at 10/9c on Bravo, below: