I'd suggest contacting Undiscovered Country Tours.
They do tours I think exclusively in California, mostly Northern California. I went on one and it was very well run and was definitely not just short distances. I thought it was reasonably priced, though you may disagree. I think they also can arrange bike rentals and I also think they're open to doing custom tours for a large enough group.
Here's all the info:
http://www.udctours.com/