I work in internet marketing. I'd say about 95% of marketers fail miserably.

Before jumping into something, you need to figure out your angle. What is going to separate your business from the other hundreds of thousands of fitness sites out there now.

How are you going to monetize it?

What is your plan to drive traffic? Forums require an insane amount of traffic just to get started. Who's going to sign up to a forum that has no other members?

You have no budget, no knowledge of online marketing, and you expect people to do everything for you for free? What's stopping more experienced people from taking your supposed good idea and creating their own website?