Hi PL,
As usual, the Info Boffin got what you wanted.
About 2 years ago, I travelled from Koh Samui to Koh Tao by speedboat, and when I was finished on that island, took a boat to Chumphon on the mainland. Chumphon is maybe 2 hours or more bus ride from Sura Thani, (where you will get the ferry to Koh Samui, arriving at Na Thon, on the other side of the island to Chaweng).
Me and my friend decided to travel from Chumphon to Phuket. It took 12 hours and cost 180 baht per person!!
It was an incredible way to see Thailand, but after the journey, I felt as if I had been raped in the ass off a ladyboy
If I was going to Thailand for a month, I would definately do the bus again.
Have fun