[Looking for Long Term Mentoring only. I believe that true value from a mentoring relationship can be realized if the two people involved are working together for a few months, as that allows them to cut through the fluff and actually communicate at a much higher bandwidth.]
I have been working with startups for the last 5 years, helping them build their core systems multiple times and shaping technology and culture there.
I attribute a lot of my learning to seeing my code and systems running in the real world for more than a year, many times doing things they weren't built to do. I believe that you lose a lot of potential learning if you don't get a chance to see your code evolve over time because that is what actually tells you if you designed/built the systems that were easy to change with requirements. How else will you know if the tradeoff you took or the assumption you made was the right one? Or if it was wrong, how did you miss it?
I have also been fortunate enough to lead teams and people, and I have learned to listen to not only technical problems but also people problems: feeling lost in your career, motivation troubles, office politics etc.
I find myself sharing and mentoring a lot of people I know personally, and seeing them succeed, with these learnings I have had working at high-growth startups. I think I can help you too :)