Your not really doing anything wrong. You will always hit a "Wall" when trying to lose weight or exercise, wheather it's improving fitness, gaining muscle or trying to lose that extra pound.
Keep doing what your doing, and only if you feel up to it try doing just a little more exercise, if walking do half a mile more than you usually do, but don't over do it. The body very quickly gets use to doing the same routine every day so try and change it slightly.
My hat is of to you that at 70, your still willing to do something about your appearance. Don't let it get you down, do something that makes you feeling good, I think that is just as important.
Good luck and best of wishes.