I think one of the most valuable aspects of face-to-face time is that the mentor can challenge you, or lead you in directions it might have taken you longer to

I think one of the most valuable aspects of face-to-face time is that the mentor can challenge you, or lead you in directions it might have taken you longer to get there or you might not have even thought of. Cf the Socratic method, which I think far too many classrooms eschew (I can't count the number of times I've fallen asleep in dry fact presenting lectures). That being said, sometimes hands on guidance (especially for more corporeal pursuits, such as playing a musical instrument or weight training), can save not just wasted practice time, but correct bad form before it does possible physical damage.