Even if you just use martial arts to enhance your fitness training, it would be still be helpful to train in a traditional discipline that has an instructor that will correct your technique. If you just go to something like a kickboxing workout class, you will definitely get a workout, but less 1 on 1 correction of your form. Additionally I would take a discipline that is more in line with popular fitness classes. A few styles that have training that are more in line with modern fitness are:
Tae Kwon Do
These styles are a little more popular in regards to competition and are a little more oriented with modern sports fitness in mind. From there, knowledge on physical fitness enhances your understanding of the martial arts and vice versa. I used to take a sports nutrition class in college and that helped me manage nutrition and exercise. I hope this info helps!