The bus ride from Bangkok to Ao Nang is hefty. Luckily, the cabin is very comfortable as it's air conditioned and has a bathroom on board. What makes the journey much more manageable is the many stops it will take so you can shake out your legs.
There is 1 operator that run from Bangkok to Ao Nang, with 1 departure per day. If you decide to take a bus-ferry, you can take the Standard option.