Best advice I can give with regard to speed and accuracy is what works for me. Input the riff, run, scale or exercise in midi and put it on a loop. Play along with it at a slow pace then increase the speed by 5 bpms whenever you've mastered the pace. I use this method when I need to practice something I can't play and it works very well for me. Can't advise on instructors though cos I've never had one! ;-)