I don't have much time to make some tutorial (unfortunately), but after you grab one part of the terrain (it's about 2x2 km), just click "Add more imagery", move the square for the next location and grab. There are some ways what to do next - one is to explode everything you have just got and intserselect together. Then you will get one object finally (with couple textures), but polygons will overlap. They did some better now and it's not really visible too much. So, maybe don't merge these squares, I think...
Then export as 3ds (since you have SK 8 PRO - it has 3ds export), and xpack the 3ds file adding textures that Sketchup creates. They are small and really low resolution.
I did it some other way - did not explode, export as 3ds and open in 3dMax - then delete unwanted (overlaping) polygons, make some corrections and merge vertices. After that I mapped the whole thing with new textures. So, maybe the terrain was not real visually, but looked much better.
If you have problems with 3ds export from SK8, save the file as SK6, open in 6 and export as kmz4. Then the procedure is as known for SK6 - copy the exported kmz, rename to .zip, extract - and load into xpack as collada. Barbje_Keller did a nice tutorial for it in the past. (SK 6 you can find in RD's Downloads).