LightUp provides two file formats for storing the light-baked geometry - FBX and DAE. You can choose which of those that you want to use by selecting Tools-LightUp-Settings-Default Model Format from the SketchUp menu and choosing either "Autodesk FBX" or Collada DAE". Save the rendered model to the selected file format by clicking on "Model" in the "Export Lighting" section of the "LightUp" dialog box that appears after rendering is complete.
For both FBX and DAE there are a number of related files that are exported, not just one file, so it is best to save to an empty folder. All files must be in the same folder for the primary FBX or DAE file to be read correctly.
Although SketchUp Pro does read the DAE format, I have not found it to be able to correctly read the DAE file exported by LightUp. There may be some way of "laundering" it through another application, such as Rhino, but I haven't tried it.
Even if the light-baked model could be imported back into SketchUp, the result might not be what you want. There wouldn't be any reflections, since reflections are view-dependent and aren't saved with either of the formats. Also, any change to the model would destroy the integrity of the rendering.
Something to know - As LightUp renders, it saves intermediate rendering results in a file called a "Cache". When re-rendering, even with a re-opened file, the cache results can shorten rendering time, often by a considerable amount. Likewise, if you render and make a small change to the model, then re-render, the re-rendering will go more quickly than the original render. (Note that the "Ignore cache" checkbox must be unchecked for the rendering time to be shortened). You might experiment with this to see how it works.
btw - An hour is a very long time for a LightUp render. Try increasing the size of the "Resolution" in the "LightUp Preferences" dialog box. Also, if you have a large piece of terrain, such as grass, right-click on the material and select LightUp-Edit Material and check "Always fully lit" or "0.5x Resolution". If you check Always fully lit, the rendering engine will ignore to material and just show it as the color of the original material, which might look ok. If you check 0.5 Resolution the rendering engine will reduce the rendering quality for that material (which may not matter), but speed rendering of that material by about 2X.