I hiked this area recently and it was really beautiful, it is one of the foothill area parks of Mt. Diablo. The Stage Road route is a moderate climb, not too hilly but an approximately 4.6 miles round trip (took me 90 minutes total when I hiked it recently). Part of the trail goes into Mt. Diablo State Park, and we will cross a few rocky, muddy, low creek beds. We will double back where Stage Road intersects the Burma Road trail.
You need to have good hiking boots or sturdy shoes, water and sunscreen. Ken suggested that we can meet in front of Wallgreens at the Encina Grande shopping center at the intersection of Ygnacio Valley Road and Oak Grove Road in Walnut Creek. After the hike we can consider going to Rocco's Italian restaurant next to Walgreens if people are interested. We will need to meet here as the trail-adjacent parking lot at Castle Rock cannot handle that many cars and will need to carpool to the actual trail site.
Address for the Walgreens is 2923 Ygnacio Valley Road in Walnut Creek. The Encina Grande Shopping Center is right before the intersection of Ygnacio Valley Road & Oak Grove Road, on the right hand side.
Address for the park is 1700 Castle Rock Road in Walnut Creek.