We are going to go to take Bart from Walnut creek to Embarcadero, then walk to Golden gate park (google says its only 5.4 miles. No biggie.). From there we can explore for around a hour, go to AFA, the bonsai shop, then AT&T park. We will most likely take a bus for half of it. Not all of it is going to be walking. Basically, its going to be a big loop. We are going to see the botanical gardens at the park. If we don't have time to go to AFA, we can always go after the game.
I think you've grossly underestimated the time required to walk 5.5 miles in the city. And then AT&T park is back on the opposite side.
In a time crunch link this, I'd be taking a taxi. For the two rides you need, you'd total out at 40 bucks, likely.