I dated a guy who was just not into being as active as I was. Honestly now I could say that he was a bit of a couch potato. Idk.
Anyway, It's hard to support a partner who says he or she wants to do something but never follows through or drags there feet and you are trying to support them
I think it's great you are trying to support your partner. I often told the one I tried to support...that it didn't matter how good you were...or how hard you went...or what you picked...you just had to go.
Maybe a gym isn't the right fit for her? Would she rather do pilates or crossfit or Zumba or just go to a squash court and hit a ball around?
I know that if I don't vary up my workout routine once in a while I lose excitement for it. And Its much easier to stay fit if you're excited about the classes you pick and self excersise routines . Maybe that could help her?
Ultimately if she doesn't want it, there's nothing you can do. Then you'll have to pick staying or breaking up. Unfortunetly.
Good luck!