Hi Anya,
Mt Diablo is a great hike in the area. If you want to venture out, I enjoyed some hikes at Skyline Wilderness Park in Napa, and you can get some double-digit mileage in with respectable elevation gain as well. Plus, you're so close to some great food, wine and beer, which always helps my training programs
Coit Lake at Henry Coe Park is a hike I have not done, but I know others who have. They all did it as an overnight (I think 11-12 miles each way), but I don't know anyone who's tried as a day hike. I'm sure it's a workout!
If you don't mind driving even further, there are some terrific hikes in the Sierra. Mt Tallac at south Lake Tahoe is a great day hike. When we lived in the American River area, we did lots of hikes there as well as north of I-80. Some beautiful landscape, and we never encountered many people outside of the immediate Lake Tahoe area. That's a bit of a haul from the Bay Area for regular hiking, but let me know if you'd like any recommendations!
Good luck and have fun with your training!
