Here are a few examples of course layouts:
To create the course, I use three curves. The primary center curve is used to track where the pod is on the course. While two outer curves create the canyon walls. By moving all these independently, I can easily adjust the width of the canyon creating narrow slots or larger open areas. Canyon steepness is adjustable too.
For the easier levels, I want to use more straightaways, but I run in to a problem. Assets are turned on and off as you fly around the course. The further forward you can see means the more assets need to be turned on. Have to be careful here, because turning on too many assets at once can really drop the frame rate.
Once the outlines are done, I can adjust the terrain heights. You will never be able to fly higher than the canyon walls, so by increasing the height of the terrain relative to the canyon wall, I can limit the flight area and squeeze the player a bit.