Meanwhile only the trainride station to station between Menlo Park and SF is 45 minutes minimum (6 stops), assuming some commute time to the Menlo Park station and a 10 min walk after the train arrives, 50 min is cutting it short.
The commute from Mission gives you a variety of options, you could even walk it if you have the time (personally, I used rollerblades when I lived in the Mission and worked maybe half the way to South Park).