There are firms that specialize in selling fitness clubs and businesses. When we sold our in-home training business in Chicago a few years back we had consulted with a broker who was specializing in these type of deals. I can’t remember his name nor his firm’s right now, but you can do a search online and ask around.
They usually do all the required preparation and analysis of your company (profit, running costs, assets, etc.) as well as the rest of the market so they can come up with a fair value. Since you are 2-3 years out, this is the best time to start the process because it may take some time to find someone who will give you an amount that is satisfying to you. I hope this helps.