Hey! I'm 49 (turn 50 on July 4th), and have been up and down the scale like you! Still struggling at this time at 312! I agree wholeheartedly with what everyone's saying in this thread, and just wanted to add that I understand about the sore knee thing. It makes it extra hard to get going on a walk when it hurts!
Anyway, my friend is heavy, too, and she purchased a recumbent exercise bike. It's the greatest thing since sliced bread (as my dad likes to say)!
She has rheumatoid arthritis especially bad in her feet and knees and it has let her be able to exercise faithfully with very little suffering.
Just another idea...