Since time is the most precious commodity on this earth, invest the time at the front-end so that you can reap the benefits on the back-end. This means putting in a bit of sweat equity and not getting paid today. Rather, you'll get paid somewhere down the road. And you'll continue getting paid whether you keep building that passive income stream or you stop. It's obvious that this is the preferred route, but clearly the road less traveled.
A fitness site can have numerous revenue streams. Create healthy eating plans and recipes and lock them away as premium content. Promote and sell a fitness training course, eBook, or a series of videos. Or advertise your own personal fitness services on your site. Topfithub is a good example of a fitness site with decent video content alongside product reviews.
The problem with affiliate marketing, like many other home business options, are the so-called gurus and get-rich-quick programs that suggest affiliate marketing can be done fast and with little effort. Odds are you've read claims of affiliate marketing programs that say you can make hundreds of thousands of dollars a month doing almost nothing ("Three clicks to rich!"). Or, they suggest you can set up your affiliate site, and then forget it, except to check your bank deposits.