back to thialand

We plan on traveling from the Sihanoukville area along the coast back into Thailand, we then want to make our way back to the Kho Lak area, do we have to go back to Bangkok and then down to Kho Lak or is there another route? we plan on going by train or bus due to cost. Any advise on the best way route would be most helpful

thanks in advance

bourjois