For anyone considering this option as a way to make money online from home, just know that if you are going to do this, it should 100% be something that you are overly passionate about. You are going to have to put in a lot of work and a lot of time before you see the results ($$$). And even when you see the results, you need to maintain consistency. Make sure that if you want to blog, you're willing to have delayed gratification, putting in the work first.
If you’re a fitness buff and have the right combination of charisma and business sense, working as a part-time online personal trainer can be both physically and financially rewarding. Once you build up a reputation and client base for yourself, it could easily turn into a full-time endeavor for you. Check out this interview with several fitness blog owners who are making a living online, from MonetizePros. As well, I'd recommend checking out this resource if you want to take this business idea seriously and get started with a business plan for your personal fitness trainer business today.