This will likely be a problem if it becomes a long term relationship. I was married to someone for 23 years who did not share my desire for fitness. About all she ever did was occasionally walk and diet in the spring to not look fat in a swimsuit. I never complained or made her feel bad but it was frustrating.
I have been an athlete/runner and worked out since I was 14. I've always stayed fairly fit. She never did a sport in HS or college and has never been active to this day. I didn't think it would matter because I loved her and she was relatively slender at 22. By the time we hit our mid 30s and she had some babies, everything changed. She's not obese but very mushy and unfit. We're divorced now and I'm dating a 43-year old who does yoga, pilates and strength training. She's very nice to look at head to toe and she also appreciates my desire to stay fit and encourages me. Probably sounds shallow but man it's nice when the one you love also has and works to keep a hot body.
Inactive people usually get plumper as the years go by. People who were not athletes as kids or young adults will sometimes catch the 'get in shape' bug and start running or some such, but chances are slim that your non-exercising SO will ever be as committed to fitness as you are.
FWIW.