+1 on the above. Your day SB-Carmel is just too much.
The Hearst Castle tours are each several hours. You leave your car in a lot at the base of the hill and take the shuttle bus up to the castle for the tour, which takes longer than most expect. I also agree with the above suggestion to book ahead. The tours are very popular and sell out often. There is no "independent tour" option. You have to go on one of the guided tours.
I'd suggest spending the night before Hearst Castle nearby and trying for a morning tour. After that, you could take the afternoon to drive up through Big Sur and on to Carmel. You would be well served to plan on going slowly through that stretch as the scenery is amazing. Stops along the way will reward you with wonderful views.
Overnight spots along the central coast before Hearst Castle are found in San Luis Obispo, which is not actually on the coast but is well-situated to access the various wine tour areas; Pismo Beach, Avila Beach, Morro Bay, Cayucos, Cambria and San Simeon, which is closest to the castle. Your preferred type of property will dictate where to stay. Higher-end properties may be found in Avila or Pismo and Cambria. San Simeon is primarily inexpensive chain hotels. There is one seafood restaurant in San Simeon of which I am fond, called the Manta Rey (sic.) If you don't mind a night in a lackluster motel, dinner there might make up for it.
In another thread I listed several places to go in this region and a few web sites that might be helpful.
http://www.flyertalk.com/forum/calif...alifornia.html
The central coast is an area to savor, not just to speed through.