What gear ratio should I use on my bicycle?

I recently purchased a single speed bicycle, mainly for fitness purposes. I loved the difficulty of it when I first got it. I was guaranteed a good cardiovascular workout each and every time I rode it. Now that I've gotten stronger, however, I can only really achieve the same workout with frequent sprinting which isn't all that practical. I've been looking at information relating to gear ratios but am totally confused. My tire size is 700 x 23, my cranks are 177 and my front and rear gears are 46 and 16. I'd like for pedaling to be substantially more difficult, but not so difficult that I risk injury, or that the fun is completely eliminated for lack of being able to reach any appreciable speed. I'd also like to avoid going through the trouble of changing the gears only to find that my new setup won't work for some mechanical reason I'm not yet realizing, or that the increase in difficulty is negligible. Any recommendations or advice is appreciated.