this will depend on the interview setting. Since I also live in Raleigh, I am familiar with the local gyms. For example, if I were invited for an interview at Rex, I would probably go with a sporty suit, not sneakers but regular shoes. I have seen some of my fellow instructors go for an interview with Lifetime Fitness in nice slacks and a sports top.
If you anticipate being in a gym setting, I would dress for the job you are interviewing for. If you apply for a trainer job, dress as a trainer.
You can also visit the prospective place beforehand and get a feel for the place. See how all the employees are dressed from the front desk to the training staff and be just a little more ‘dressed’.