before YC, before my CS degree, and just barely after graduating high school, I spent two years in La Rioja and Cordoba, Argentina as a missionary for the Church of Jesus Christ of Latter-day Saints (mormons).
I have had a LOT of people ask me why I did it, what it was like, etc. etc. and I won't get into all that here.
But I do want to say that going door to door, serving, talking about Jesus, and getting rejected constantly instilled in me a level of grit that would be impossible to replicate through any other experience.
The best founder training in the world is working every waking hour on something you deeply believe in, with near-constant rejection.