Apart from the texture stretching bad it seems ok. I found adding more vertices works best
in this case I would add more vertices just to the front/back keeping model size low.
I like to use 3d Coat for all my cave texturing as it gives me an option to vertex paint, if your on a budget then 3d crafter can vertex paint but it is somewhat limited.
as an idea I would create ground textured tiles to cover over the tunnel roof so you don't have to try and fill or cover over to try end blend in to the raised ground around the tunnel.
Tip: use large detailed textures, at least 2024x2024 that will affect texturing on large models/meshes
I have posted a few samples of some stretched texturing leading up to finished model
I have noticed in your models their are a few broken poly's so I have welded them up in the entrance model and textured it with a new texture, have a look and see what you think.
oh also you had no diffuse/normal and specular texture maps. so I created them for the new texture I have added.
Nice tunnel and good to see your doing well on this subject.
