> I think fitness is a bit of a unique market because, in my experience, people are more willing to spend money on it compared to other types of software.

It is one of 3 markets that are like that: health, dating/relationships, finances.

"Fitness" as a concept pertains to the "dating/relationships" market and is therefore linked to the strongest motivator ie. gettig laid. It is also linked to health and for some it helps with their professional performance. So yeah, "fitness" hits a sweet spot there.

But really, have a seriously useful app in either one these three and you can build a business around that.