Hi there, welcome to the Babylon community! Your question about walking simulators is super relevant, since @syntheticmagus is working on “golden paths”, i.e, helping developers produce certain kinds of experiences, and a walking simulator is on the list of future golden path posts. 
But for now, there are a few discussions about dynamic/endless terrais that might be useful:
Dynamic Terrain extension, How to make infinite terrain? - Questions - Babylon.js (babylonjs.com)
Strategies for large terrains - Questions - Babylon.js (babylonjs.com)
Divide terrain in sections with specific data - Questions - Babylon.js (babylonjs.com)
High resolution terrain - Questions - Babylon.js (babylonjs.com)
I found them by searching for “terrain” on forum search.
Hope these can help a bit!