I want to create an underground hallway. I added a cube in an empty world, resized it along the Y axis, downloaded concrete textures and put them in nodes. But the texture looks stretched. Is there a way to fix this?
The texture is distorted, because you scaled your mesh, and you're not using a UV map.
1) Apply object scale
Because you scaled your object, your scale is probably messed up.
Just go to Object->Apply->Scale
2) UV unwrap your mesh
You need to UV unwrap your mesh, so the texture is properly positioned.
In Edit mode, select all through Select->All
and then go to UV->Smart UV project
3) Connect your UV map
Add a Texture Coordinate node, and use the UV output.
Add a Mapping node, to control the scale and position of your texture.