For starting off, I'd try to work on overall anatomy, and then after a while nitpick the parts you are weakest at (or simply want to get better at) and work from there. It makes no sense to be able to draw really nice arms when everything else is like a fat stick figure versus having a decent 'everything' and then working specifically on improving your arms.
For starting, I wouldn't time yourself. Focus on anatomy and once you are comfortable with that start doing the quick messy gesture drawings. I like doing the 30 minute class because it builds you up and ends with a 10 minute sketch. Plus it doesn't take that long and is a great way to motivate yourself to work on other pieces.