This is great advice. A lot of people are busy but there are some who are more than happy to be mentors, it's just a matter of finding them. Could become a numbers game which is worth it imo.
I've found it to be incredibly rewarding to let the relationship develop organically. I try to have insightful conversations on Twitter and Linkedin. A very small portion of these conversations turn into strong relationships. Some people are on the lookout for "rising" talent and would initiate the relationship if they see you as one...