Unfortunately, Jakob is correct that SketchUp has been "updated" so that you have to have the Pro version to get terrain data, and even worse, the data is way lower resolution than it used to be. Basically, SketchUp is worthless for this now, IMO. Which sucks because it used to be pretty easy/good quality.
The only way I know of to get good quality terrain data now, aside from going out there with a drone, is using DEMs. There is some helpful discussion on it here: viewtopic.php?f=11&t=8845
Beware, it can be confusing AF. Definitely read down the thread, especially Haggvist's posts.
First you have to find the DEM (height data) and ortho (satellite imagery) that covers the area where your track is located. I don't know about international sources, but for the US, the National Map is very useful: https://viewer.nationalmap.gov/basic/#startUp
(Search for "Elevation Products (3DEP)" and "Imagery (NAIP)" in the area of your track.)
Download the highest resolution DEM and ortho available and open them in a program such as Microdem or QGIS (I use QGIS since it is available for Mac). In these programs, you should be able to align your height data and satellite image based on the coordinates embedded in the metadata of the files. Then, zoom in to the part of the map that you want to use in game, and export the height data as 16 or 32-bit grayscale, and export the satellite image as color. You can then open the images in Photoshop/GIMP and convert the grayscale to 16-bit (if it's not already), and proceed to work on them as normal.
Once I get a better grasp of exporting things from QGIS I'd like to make a more detailed tutorial, but I haven't quite learned it 100% yet.
Hope that helps! Unfortunately since the new SketchUp it's a way more complicated process, but definitely still possible.