From the Marina, your best bet is to take the 30x which is an express bus that stops along Chestnut Street, and then again in the Financial District. It's about a 20-30 minute ride depending on your stop(s) and traffic. The downsides are that it can be very crowded at peak times, and the last bus out in the morning is 9:12am, and the last express bus home is around 6:30pm (which I find to be a bit early). If you miss the last express bus home, you can take the regular 30, but it takes about 35-40 minutes.
I walked home the other day, but it was about 3 miles, so probably not a round trip you want to make every day.