FWIW, SF resident here. My wife and I on our Co-Motion Speedster tandem have done a number 2-3 day loops from SF in wine country and along coast. We're 62, in average shape, and sometimes ride laden with touring/camping gear.
We find the best way from SF to get to Napa or back is to take the ferry from SF to Vallejo.
Here's a possible *very roughly described* route for you to look into, leaving a lot of detail out, including where to overnight (we always camp). I wouldn't hesitate to do it as a 5-day trip, especially if credit-carding:
-Ferry from SF to Vallejo (lovely 1-hour trip through Sf Bay)
-Vallejo to Napa (using ridewithgps, we did this a few weeks ago and it was busy some places but really fine)
-Napa to St. Helena (great scenery and mostly good cycle lane on Silverado Trail, though it can be a bit busy; avoid Rt 9 and 128)
-St. Helena back on Silverado Trail to Calistoga. Jimtown Store is cycling classic. If energetic, take a loop from Silverado Trail around Pope Valley and back.(loop described in book below)
-Calistoga to Geyserville then over to W. Dry Creek Road (lovely narrow paved road with little wineries; never busy the 3 times we've cycled it; two great loop routes in this area described in book below)
-W. Dry Creek Rd. to Westside Rd. to Healdsburg
-Back on Westside Rd to River Rd. to Guernville (stop at Korbel, buy bottle champagne, drink in garden!)
-Guernville to Pacific Coast Highway One just south of Jenner, then south to Bodega Bay (highway 1 can be busy but early morning on weekend we found it just fine)
OR
-Gurenville to Monte Rio then south on Bohemian Highway to Bodega Bay, mandatory stop bakery in Bodega on the way.
(we did a loop one day from Schoolhouse Canyon campground near Korbel to coast to Bodega Bay to Bodega then Bohemian to Guernville and ended up at Korbel for champagne. A fabulous mix of terrain and scenery.)
-Bodega Bay to Pt. Reyes Station (out to Pt. Reyes if you're energetic and don't mind headwinds.)
-Pt. Reyes Station to Stinson Beach to Muir Beach
-Muir Beach to Sausalito
-Sausalito to Golden Gate Bridge to SF (or take Ferry from Sausalito, but you really want to take GG Bridge)
For great routes in various sections, especially to avoid busy roads we have:
-Taken inspiration from this book:
https://www.amazon.com/75-Classic-Ri.../dp/B00IUP9NPK
-Used a paper cycling map of the entire North Bay (don't have it with me now so don't know the maker) with good indications of incline/declines and routes
-Planned routes using ridewithgps.com app
-Checked out satellite views on Google maps
In any case, if a cupla oldsters like us can do it, you can too, with a little research, as none of us ride the same.