Josh,
Best to work for that one fitness center and learn as much about the business as you can.
What's your fitness niche
Could be MMA type training, general equipment focus like treadmills, weight-lifting etc etc, yoga or specialized athletic training. That will help determine your start-up capital cost.
Be prepared to work 120+ hours per week.
In the fitness industry, your main goal is "Sales" ... annual/lifetime memberships. You'll need to hire an aggressive sales force.
Answer the above questions and it'll give you a start to your business plan write-up. Best to google search for samples of business plans relating to the health industry. After that, you'll need to sell your business plan to investors. If the potential is as good as you think, you should have investors willing to risk $$$ but be prepared to give up equity.
Up here in Ontario, former NHL player Gary Roberts is doing an amazing business specializing in training hockey prospects using unconventional training methods and emphasizing organic nutritional supplements. Google his name for more info.