Do you like to read books? When was the last time you said "Oh My God! This writer is totally cool, but you know what, Im going to put him back on the self, coz, you know, he doesnt have a degree in English?"
Just like having a degree in physical education is not required for winning the Olympics, having a degree in English is not required for writing a successful book.
Writing, like sports, is a very practical thing. Sure, you may go for coaching, ask for help on how to improve your weaknesses, but at the end of the day its not your bookish knowledge, but your practical experience that saves the day. So practice a lot.
Finally, looking at this survey:
By far, the two most popular choices were conventions and writers groups, both of which were reported by more than half of our novelists. The least popular choice? The graduate degree in English/Writing.