But afterwards: into the Mission district in search of burritos. Mission is odd: Mission Street itself is run down, heavily Hispanic, and often edgy (and there are certainly blocks you wouldn't want to walk on after dark). But move a block or two West and there are pockets of white trendster/yuppie enclaves: coffeeshops and loft-living furniture stores.
La Taqueria (Mission & 24th): another one Melinda remembers from family excursions, but also a long-term favourite of reviewers. Excellent carne asada burrito. I had mine plain, so it was mostly meat and salsa: very very good, definitely best burrito yet. And we were still hungry afterwards, so down a block to one recommended by ChowHound:
El Farolita (Mission & 23rd): more of a hole-in-the-wall dive than La Taqueria, but cheaper with it. Another carne asada burrito, which was good, but not as good; maybe we should have tried the al pastor as this post recommends?
To walk it off, we walked up Mission onto Market and along to Embarcadero BART: quite a trek, but does joining BART 5 stops up save much on the fare back to Walnut Creek? Not really: 24th & Mission to Walnut Creek is $4.10; walking 3-and-a-bit miles saves you precisely 10¢.
(Future reference: County Connection Route 104 in Walnut Creek is free to ride, and runs between BART and Broadway Plaza: saves a bit of a walk getting home. Doesn't run evenings, though.)