I recently did the walking food tour of the Mission with Mark and couldn't rate it highly enough.
The walk itself and history of the area was really interesting, and showcased a district I didnt see on the bus tour of the city. There are hundreds of murals in the district, alot tucked away down non descript alleyways you might not otherwise take a look down with some insights on the murals and what they depict.
There were 5 well spaced food stops on the tour, and our group was certainly well fed by the end of it, without being uncomfortably full (little bags were provided to store leftovers for those who had them). None of the food was too spicy, and alternate options were provided. The 5 stops were all delicious, covered sweet and savoury and included places that had been in the neighbourhood for years.
Mark gave some excellent recommendations for restaurants for trying. All in all, I would highly reccommend this tour to anyone staying in San Francisco who wants to see (and eat!) something off the beaten trail.