It's an interesting idea. There is a possibility though, that Hacker News itself is the only HN-type site that can be successful.
What makes HN work isn't the software, it's the behaviour of the community, which vigorously votes up and flags spammy or off-topic posts. I imagine that a community of tech-minded hackers are the most suited for this activity. I imagine that a large majority of those visiting a fitness-related site would be less tech-savvy and logically-minded.
You are very likely to get a LOT of spam from advertisers, and if the community does not moderate it then you'd need super-users to do it. Maybe digg would be a better model?
