Inspired by Jaye Wiley’s advice over in the fantasy forum, I had a go with constructing flexible roads and rivers with decorating caulk. I purchased a brown caulk and gun from Amazon, and squeezed blobs out onto greaseproof paper.
Wearing gloves, I smoothed the caulk out flat into the desired lengths and shapes for road and river sections, then sprinkled some play sand on top and worked it into the surface.
After a few hours drying I peeled the sections off and flipped them to allow the rear side to dry.
After drying I painted and dry brushed as desired.
Very cheap and easy to do! The sand sticks really well after you shake the excess off, and the pieces are heavy enough to lay flat and not shift around during play.