This is so fascinating - and something I've never heard about before. I've always thought I had very strong will power, but now that I think about it (!) I do have very strong will power when I apply that skill, but I often talk myself out of it. For example, "Actually it's best if I eat that marshmallow now, because then I'll have a longer time to digest it before bedtime..." So you're absolutely right - will power is mostly about practice and training.