Well, depending on how in depth you want to get, you might want to try looking at some real world GIS software (QuantumGIS for instance) rather than things like Campaign Cartographer which is really more of a specialized graphics editor (which isn't necessarily a bad thing as it is a lot easier to use than a GIS)
Working with true spherical geometry with only graphics tools is hard. Working with spherical geometry even with the right tools can be hard. So if you want to make your world really work as a sphere, you are going to have to learn a bit of theory behind geography/cartography and spherical geometry.
Most of the people on the guild don't bother with this and if you don't know how to do it right, then you probably won't notice it being wrong. Many people get by with maps where their world is effectively flat or cylindrical rather than spherical (let alone the spheroidal models we use in real life). Such maps will often look blatantly wrong to anyone who knows anything about geography though.
The zooming in thing you want in particular is difficult. If you want a decent map, you need to pick a projection suitable to the particular extent of the map. A projection suitable for a global map is not going to work for larger scale maps. (In cartography speak "scale" is what you might think of as "zoom" so "large scale" is zoomed in). This is because all projections of the curved globe onto a flat map will cause distortion. If you have a restricted extent, you want to pick a projection that minimizes distortion within that particular extent. So just zooming in will cause problems if you zoom in on a distorted area.
Web maps like Google Maps cheat by using the Mercator projection which gives passable results at large scales but is only really a good choice at smaller scales for certain kinds of navigational map (which is what it was designed for). it was widely used for world reference maps for a while, but wasn't a good choice. It's a really bad choice for maps at the scale of continents or large countries particularly in high latitudes (It really screws up Canada and Russia for instance.) The reason it sort of works is that Mercator preserves angles, while distorting area. Over extents where the amount of distortion doesn't change much, it looks pretty close to what a tailored projection would have given you. Over larger extents though, it is quite obvious.
One option somewhat simpler than all out GIS is to use "G.Projector", a free Java tool made by NASA. It can take a map in a Normal Equidistant Cylindrical projection (also known as Equirectangular) and project it into a wide range of other projections. Equidistant Cylindrical projections aren't a good choice for finished maps as they have particularly unpleasant distortion. However, you you can lay out your world using this projection, and raw the appropriate distortion into it, you can then easily reproject it and then make finished maps from that. It's not as flexible as with GIS, but it's a lot easier to get started. The big downsides being there's no way work backward from other projections, and that it doesn't support vector data.