For the distance hiker, Sourdough makes an excellent loop daytrip. Start from the town of Diablo, hike to the lookout, then down the Pierce Mountain trail to Ross lake. Follow the lakeshore trail SW to Ross Dam. Cross the dam, walk the service road down to the powerhouse, then cross a sturdy foot suspension bridge to the woods on the N side. Hike steeply up, then level off on a good horse trail that takes you all the way to Diablo Lake Resort. A mile of service road takes you to just past Diablo Dam, where there's a trail beside/below the incline railway into the town of Diablo again. Walk to the tourist bathrooms, use the payphone to call for an ambulance to take you the last 500 feet to your car. Total distance 19 miles, from my map measurements.
Notes: There's no water from when you leave the ridgecrest just E of the lookout, until just before Ross Dam. The other two miles of lakeshore trail are 400 feet above the lake. By the time I reached water I was too dehydrated to care about hygiene. I just drank out of Ross Lake. (No ill effects.)
You must cross Ross Dam. I tried a trail that looked like a shortcut at the NW end of the dam, but it dead-ended.
Don't cross Diablo Dam. Stay on the road past the end of it to the trail down to town.
Travel light. Pick a sunny day in late Summer so you can go with just a fanny pack and running shoes. But don't call me if the weather turns sour.
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe