After modeling the slope that your drawing describes I used that model to project the pathway onto it.
I tried using a simple bezier curve, drawing in the middle of your pathway and adjusting the handles of two points to make it smooth. Then I converted the curve into a mesh. After this I experimented with the following modifier stack:
**skin
shrink-wrap
boolean
subdivision surface**
This was the result after using solidifier on the slope which I had modelled from a plane. It still took a bit of time to experiment with the settings. I also tried doing a boolean on the slope itself in order to sink the pathway down. It produced good results but required more objects.
Option B: If the pathway is very complicated, one option might be to consider how to bring in your pathway as a single .SVG. However for this you may need Inkscape or Illustrator to do an image trace.