PathAvoid is really not meant for macro-scale endeavours and is more for micro-scale (room to room) tool. This is more an underlying problem with how the base game's pathfinding works and I welcome you to try different values and see if something else works.
In-depth for Path Avoid's hints: each tile can have a 0 to 255 value of 'difficulty'. W/o this mod every tile has a value of 0 and as things like grass, mud, trees, rocks, etc get put on a tile the difficulty goes up. This mod increases the base value to 10. Preferred becomes 0. From there dislike, hate, strong go 50, 150, 250 (iirc just going off memory). So surrounding the non-roads with 'strong' will probably give a stronger 'hint' to the pawns to use the roads but the pawns could very well ignore that too (especially for longer distances).
Workaround: In the mod's options you can click 'advanced' and you can put in your own values for each path avoid type. I have not personally tested it with other values with this release. Setting Normal to 50 might give a stronger hint to the pawns. Again though, especially for longer distances, your pawns will probably ignore path avoid.