Hi holspen, welcome and thanks for the question.  I think most middle aged people have the same prbolem..a belly that just won't go away no matter how hard we try.  All your efforts are giving you a good core , but to burn fat you need to do aeoribic exercise.  A good brisk walk, gently jogging or anything that gets your heart rate up for a 20 minutes or more, which your doing on the treadmill.  Intervals may help too...good fast for few minutes, then slow for a few minutes alternating for however long you spend on the treadmill.   Another thought it could be genetic...i talked to my dr. about this and he ask "What did your Mom look like?"  when i told him a bigger version of me he said "there ya go...ya can't fight gentics or pick your relatives."  Sorry i can't be more help, have you talked to the gym people?  They may have suggestions for you.  take care...phyllis