I find juggling many special interests hard at one time. So...I'm in the same boat with you: for the time being, my business is my main special interest. And no, it's not entirely healthy, so I try to make sure I get some free time every day and do other things I enjoy (some of them my former special interests that I have previously devoted more time & effort).
I do feel like, though, that having several special interest- level intense interests in my life would be exhausting. This one is fine, as long as I try to keep a healthy balance.
In general, I have just sort of stumbled upon my special interests. Read a little and then become more and more excited as I have realized how much fun there is hidden.
So, IMO, having just one special interest is not bad- as long as it's not the only thing in your life. Work is still work and no matter how engaging it is, there is -and needs to be- more to life than that.