Diane and Greg,
It's just a quick taxi ride over, (about 5 minutes). There are plenty of taxis waiting in line. Just make sure that you tell them prior to the trip that you just want to go to Muni, and find out the rate. It shouldn't be more than $10.
When you get back to Muni, there will be taxis there also.
Enjoy your trip, but remember that a day trip is never enough!!