Congratulations on your progress and obvious hard work!!
I agree that you need to start slowly and progressively up your resistance. I would say look for a book or other program that has everything laid out for you. Make sure the program description matches your goals. With a bad back, you will probably need to educate yourself on how to modify some of the exercises...you know your body better than anyone though, so listen to yourself on this. I also have a bad back and bad hips and knees as well (and I'm only 20!), but since beginning strength training my pain has been significantly alleviated. And make sure that you are experimenting to find the correct eating plan. I'm sure you know this judging by your success, but your diet will be what makes the weight come off, and stregnth training will help you build strength and get you healthy!
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