I've been using the stationary bike and the ellipitical for years, sometimes up to an hour every day. In addition, a few years ago, I had a strict routine of 200 stomach exercise every single day. As a result of these rigorous and continous work-outs, my upper back has suffered. My shoulder blade muscles are buldging, the muscles along that area are incredibly sore and I even have some tingling and numbness b/w the shoulder blades. My physio therapist suggested that I strengthen the back muscles below the shoulder blades so that the ones above are not working over-time. Any suggestions as to how I might do this? I don't know much about weight-lifeting, but are there any specific exercises that I can do to strengthen my mid-back? Thank you in advance.

Bent over rows, one arm dumbbell rows, pulley rows, etc. These will all work your traps, rhomboids and lats, which are the ones you're looking to strengthen. Check out the exercise section on abcbodybuilding.com for some ideas on back exercises (dont worry, you dont need to be a bodybuilder).