After modeling the plane (it may in fact be completely flat, but you said side view)   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.

[![enter image description here][1]][1]
[![enter image description here][2]][2]

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.


  [1]: https://i.sstatic.net/AMRae.jpg
  [2]: https://i.sstatic.net/ZRqQ6.jpg