Hmm, not sure about exact mileage, but I would mix in some combination of the following roads: Dry Creek Road, Trinity Road, Calisotga Road, St. Helena Road (Spring Mountain), Deer Park Road (Howell Mountain), with the alternatives or additions or Petrified Forest Road and/or Oakville Grade. A route might look like:
- head south from ountville back toward Napa (I am pretty sure there is a frontage road along 29);
- take Orchard Park west to Dry Creek Road; right/north on Dry Creek Road (go warmup climb that gets steeper toward the end); l
- eft/west on Trinity Road (steeper still);
- descend off of the top of Trinity down to Sonoma Hwy (careful on descent);
- right/north on Sonoma Hwy;
- ride up Sonoma Hwy to Calistoga Road;
- right/east on Calistoga Road (on the outskirts of Santa Rosa) (start climbing);
- right/south on St. Helena Road (still climbing for quite awhile with eventual fun, not so tricky decent off of Spring Mountain into St. Helena);
- make your way east, across St. Helena to one of the cross-valley roads (ideally Pratt Ave.) until you get to Silverado Trail;
- head left/west on Silverado Trail;
- right/east on Deer Park road (you will now climb part way up Howell Mountain);
- left on Sanitarium Road; right on Crystal Springs Road;
- left when Crystal Springs Road ends; right on Silverado Trail;
- left on Lincoln Ave into Calistoga.
Okay, so the above route probably exceeds your 50-60 mile request, but what the hell; it is a geat ride. Thing you can do to shorten it would include heading north from Yountville to start and going left on Oakville grade you will intersect Dry Creek Road and just continue on Dry Creek Road as above. Also, you can simply stay on Calistoga Road instead of turning onto St. Helena Road; Calistoga Road will drop you into...surprise...Calistoga! (By way of Petrified Foest Road, I think.) Finally, eliminate the Deer Park Road climb and stay on Silverado Trail all the way to Calistoga.
Oh, and there is a group ride assocaite with a new bike shop in Napa. While it would not meet your point-to-point request, I beleive that it was described to me as a 60 mile loop, so it would probably be a good way to see where the locals ride (you also might be able to adapt it to your purposes). Bump around on Google to find the numbers for Napa cycling clubs or shops and call tehmt o get the details. Good luck.
Garth