That's something I've never been able to get my head around; not that I make time to play too much these days. For me the challenge is that it isn't linear. What I mean is I had no problem playing up a single string as you illustrate above, but would then get lost when playing in scale boxes.

Looking back I think I probably just wanted to know everything in all one go so never really practiced in a structured way. Stuck to chords ever since :D