At 75, Glen Campbell is experiencing a wave of renewed interest in his long career as an entertainer who blurred the lines between country and pop music with hits such as “Wichita Lineman,’’ and “Rhinestone Cowboy.’’ There’s a heightened sense of appreciation ever since Campbell announced last year that he’s battling Alzheimer’s disease. Campbell met with the Globe when he was in Worcester last month; his concert at the Wilbur Theatre on Thursday has been sold out for weeks.