When you said "physical therapy," I thought maybe you should do what she says, but then you added "gal at the gym" which isn't exactly a medical professional. :) ISTR that the directions from the gym person were along the lines of "machines are safer" (sorry if I am misremembering!), which, well, it's not something I personally believe.

Is your insurance/health situation such that you could talk to a credentialed physical therapist about this? Because, while I don't want to discourage you at all, if you already have a bad back being careful might be good.