Adult training courses are currently the most efficient way for adults to learn English, because if you study on your own, you may enter into a lot of misunderstandings, and you may gradually "let yourself go" because there is no one to supervise and remind you.
So in summary, the most efficient and time-saving method is to enroll in a class. The most important thing to remember is that you need to keep talking and practicing in order to have a good result. Usually their own private practice may not have a good environment to speak English, therefore, you need to enroll in an adult English speaking course, at any time someone to accompany the practice of speaking English.