Categorically, you definitely should focus on something that you are passionate about and can talk, write about without stopping. You will be the SME overtime in this area if you will.

Let's make it easy on ourselves and follow our passion and we will be the internet success to mimic down the road. Humbly speaking, I am juggling with my current four sites, gradually making progress.

Regards,

Sidney

I totally understand what you mean. I have been creating a website about health and fitness and honestly, if I wasn't truly passionate about it it would really hard to write about. I haven't always been really into fitness but now that I am, I feel confident in what I know.
The niche is supposed to be something you are into and want to become an expert about. If you really want to keep with that niche and help your own journey into fitness I would maybe recommend gearing your site towards your own journey.
People can connect with your experiences as you learn more and more and improve yourself. I would maybe write about the different things you try or new programs you have become a part of and then your results within those things.
I wouldn't try and seem like you know how to teach people about it when you aren't currently living a lifestyle that semi revolves around fitness.
Does that make sense?
Everyone loves a good story about someone improving themselves. Maybe your website should be personal and that may motivate you to become really passionate and excited about it too.