Luke began his training relatively late in life (given that the council was hesitant to train 9-year-old Anakin), spent only a few days with Obi-Wan and another few with Yoda. Anakin was trained for some 12 or 13 years, and most Jedi are trained for even longer. Luke spent a grand total of perhaps a week with Jedi Masters, how is it he became powerful enough to compete with Vader?

Is it possible that he was in rigorous personal training between IV and V and then again between V and VI? Are there answers from the Extended Universe (Legends) for this?