If you're looking for a meeting place or a good stop around ½ or halfway between Roseville and San Geronimo, you can try Fairfield, CA, which is almost an hour southwest of Roseville, or almost an hour northeast of San Geronimo.
About ¼ or a fourth of the way, you can stop at Davis, CA. This is about half an hour southwest of Roseville.
About ¾ or three fourths of the way, you can stop at American Canyon, CA. This is about 1½ hours southwest of Roseville, or about half an hour northeast of San Geronimo.
If you drive straight through without stopping at all, the total distance is 110 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 1 hour, 56 minutes.