I wouldn't recommend Ohlone Wilderness as your first trip. I did it with some friends last May, from Sunol to Lake Del Valle. It was hot and fairly steep, and while the scenery is pleasant, it most certainly isn't a wilderness, despite the name -- most of it is cow pastures. There is only one allowed campsite, which is 10 miles from the trailhead on either end; that leaves you with no flexibility if someone in your group finds that distance to be more than they can comfortably handle. You really have no choice but to keep going until you get to camp. On the plus side, it's just about impossible to get lost.
If you're at all like me, it will take a few trips to learn what's a comfortable distance to travel in a day, and it's probably better, for the first time, to plan for too few miles than too many. If you get to camp early, you can always do a bit of day hiking from there.
I will second the recommendation for Point Reyes. It has several choices of campsites and many options of varying distances for routes to those sites. No matter which route you choose, the scenery is gorgeous, and I've never seen another place with so many different kinds of terrain and vegetation in such a small area.