If you want more a modest challenge ride off the IRT, when you get down to Dublin go through the BART station at Dublin/Pleasanton, continue down Willow, turn right at Stoneridge which you take all the way up to Foothill, turn right then left on Dublin Canyon Road. This is all protected bike lane. Ride Dublin Canyon Road all the way into Castro Valley. Long, straight and no interruptions. It's a good aerobic high cadence workout, but no real suffering. I've seen lots of team training on those grades over the pass, and they pass me like I'm standing still. Peet's on Redwood Road in Castro Valley.
Nice bail-out: if you or your partner feel done in Castro Valley you can take BART back to Pleasanton and just cruise the IRT back. Heck, I guess you could also take BART back to Concord through Oakland.
A more aggressive ride is to leave Peet's for Redwood Road and go north over the hill and down into Moraga/Lafayette, but frankly that road down the otherside is too dangerous for this rider, no shoulder and too many blind corners. Lots of riders out there though....