I hate to have to tell you this, but it's impossible to give any meaningful advice without seeing the bike and you on it. Do you have a picture?
Just getting a bike fit at some shop isn't necessarily going to solve your problem, because there's just no such thing as one fit. How you fit on the bike depends on too many factors for that.
Ultimately though, if it's longer distance endurance you want rather than sheer power or speed, it usually helps to have the saddle a little further back and the handlebars a little further up (without being too close or too far from you). Back pain can be caused by improper saddle height or setback, or by handlebars which aren't in the right position for you.