As someone who will be starting a MLIS program soon, I can't see any of the 7 1/2 Habits as "difficult." I've always been a lifelong learner (only I think I have always described myself as The Geek in the Front Row of Class Waving My Hand Wildly In a Pick-Me-I-Know-The-Answer-Way). And I have been chomping at the bit to get my Master's for quite some time.
Oddly enough, the only time I thought it was "too late" and I was too old to go to grad school was a year or so after I had graduated from college (yes, I was ancient in my early 20s).
So if I had to choose a "most difficult" I would say it would have to be "Play," as going to grad school and working at the same time (and reading, and knitting) will more than likely leave little room for "Play."
I see all the other Habits as easy, but I guess I'm most excited about Teach/Mentor Others. As I've said before, I luuuuuuuve to teach people. It gives me endless joy to see the Oh-I-Get-It-Now-Look shine on someone's face.
Pictured: the hands of my teen Knit Wits