My answer to this Quora question.
From Berkeley, my favorite rides are:
(1) Up old Tunnel Road (Ashby/13), a winding ~10 mile ride that is a moderate climb most of the way. You can return to Berkeley either via Claremont Ave, or for a longer ride go up to Grizzly Peak and drop out on Centennial Ave. Both are steep — make sure your brakes are in good order!
(2) The Bay Trail segment starting from the Berkeley Marina (over the pedestrian bridge, by the Seabreeze Market) heading north to Richmond by the dog park and CostCo.
(3) The Three Bears Loop near Orinda.
The Grizzly Peak Cyclists cue sheet page is incredibly helpful and has lots of rides rated by difficulty and length. Many are hard, but not all of them. Check out their ratings system. If you want other people to ride with, their organized rides are also worth looking into.