Hey fuulin, great work on the last image on the image adjustments assignment. Makes me want to give it another shot.
You should avoid drawing from the wrist and instead use your shoulder, the larger movement range that you get by doing so, together with a little bit of practice will allow you to make longer lines.
After doing this exercise for a couple of day, my wrist also started to hurt a little bit, The solution to this was to adjust my pen pressure settings so that I don't have to push so hard to achieve different line weight or grey's.
I find it useful to hold my stylus straight up, almost 90 degree to the tablet surface. It helps me to avoid moving my wrist and allows me to draw straight lines from multiply angles.