Located about 10 minutes West of Discovery Bay off of Marsh Creek Road there are over 30 miles of trails at Los Vaqueros Reservoir.
You are supposed to either pay to get in or buy a park pass. If you are looking for this sort of trail and don't want to pay 6 bucks to ride I suggest you go to _Round Valley_ Regional Preserve which is about 3 miles West on Marsh creek road. The two parks connect with each other and with Morgan Territory Regional Preserve.
All of the rides in this area are pretty similar, essentially bulldozed fireroads over rolling hills. You can go quite fast on the fireroads but other than loose steep hills don't expect to challenge your riding skills. There is also very little tree coverage so it is very hot in the summertime.
At Los Vaqueros there are grassy parklike rest areas spread around the park to use the restrooms or to have a picnick lunch. There have also been bald eagle sightings at this location and some trails are off limits to protect the habitat of the bald eagle.
From Discovery Bay head West on Hiway 4 to Marsh Creek road. Turn Left on Marsh Creek and follow it about 1-2 miles to Walnut. Turn Left on Walnut and it will take you into the park. You can get a map at the toll booth and start at any of the rest areas. While there take a minute to check out the dam site.
Contra Costa Water Districts main web site for Los Vaqueros Watershed.