this has been a popular riding spot for YEARS. then, they built houses in half of the riding area then proceded to ban bicycles and close trails that riders built 15 years ago.
that being said, here is a map http://www.ci.walnut-creek.ca.us/pdf/map.lime%20ridge.pdf
the last time i rode there rangers had made their presence known. the most popular singletrack full of switchbacks and big berms (called mach 5) had been destroyed - large rocks placed in the middle and shrubbery had been uprooted and layed across the trail. such a shame, this was the trail that really got me into riding.
i used to live nearby, so i would ride there and do the paradise valley trail to ridge trail loop then head out. mach 5 starts at the water tower on the ridge trail. paradise valley is good to come down also, real smooth with lots of switchbacks. buckeye trail and manzanita trail are fire trails but have some tricky spots with rock gardens and gets pretty steep in a few spots.
also if you follow blue oak trail to where it meets at ohlone trail, there is a real tight singletrack there too. its not very long but it snakes through the trees and is super fun to ride fast. then you can ride through the tunnel that goes under ygnacio. on the other side of the road there is not much going on, just firetrails. watch out for hikers, i got yelled at for "scaring off the mountain lions" ??? yeah right, like they are going to come up to you and let you pet them.....
from 680 take ygnacio valley road to concord. when you stop seeing houses and start driving up a big hill, that is lime ridge. there is parking at the first stop light once you get up and over the hill. park there at the gate and there is a map posted at the sign.
http://www.singletracks.com/item.php?c=1&i=2490