The route is: Clarbeston Road station, back roads to Rosebush, then walk to Foel Cwmcerwyn, (say about 2 hours round walk).

Then back road along East edge of Preseli mountain  Carn Ingli to Whitechurch (also shown as Eglwyswen), at the North of the mountain.
Then around the West side of Preseli mountain back to Rosebush.
Then along the back roads to Whitland. (Many trains from Whitland to Swansea).
I think roughly 70 miles.