Ahh, I see what you are doing!
I rarely use the navigation panel, always numbers, so I didn't understand really how this could have happened accidentally.
To zoom, do it in increments of 5 from the render camera settings box or, if you have to use the navigation panel, set your zoom speed to the slowest speed by clicking on the first of the 5 speed bars at the bottom of the panel.
You have zoomed out to 180 degrees by using the nav' panel control. Nothing serious. Try just moving further away from what you're looking at instead of zooming out so much from a fixed camera position. Fov 60-80 degrees is usually fine for zooming out, 90-100 tops I'd say at 4:3 resolution, real stretching problems start to appear.