Not too ambitious at all, although I have spoken to folks who have done this and they felt it was too much. Mostly because that 90 mile drive between Carmel and Cambria is so winding. Since the driver has to be focused on their driving, it can be very tiring, especially if driving it down and back up on the same day.
It is about a 3 hour drive without stops, so you would have more than enough time for a few stops en route both ways.
On the drive down I suggest brief stops at Bixby Bridge, McWay Falls and the elephant seal rookery. 15-20 minutes for each stop should suffice. You can stop anywhere along the route where there's a Vista Point turn out to enjoy your picnic lunch or wait until you reach Ragged Point near the end of the Big Sur drive and near Hearst Castle. Ragged Point has outdoor seating and the property offers great views from the back of the property.
Since Hearst Castle tours only last 45 minutes now, your 1pm tour would be over before 2pm, leaving you 5 hours to get back to Carmel by 7pm. You could spend a little time exploring the outdoor areas after your tour ends, then begin the drive back, stopping at some of the places that piqued your interest on the drive down and include a stop at Nepenthe for that glass of wine.
Edited: 4:51 pm, August 15, 2012