I've done Insanity, and my only advice is listen to your body. It's a program that relies on you pushing yourself, so I wouldn't say that people quit because it's too hard, but it can be really hard on your body. I did it around my lowest weight and I'm in my mid-twenties and I stopped after a month in part because it was torture for my knees. Remember that good form always beats keeping up with the instructor, this will help you to 'land soft' when jumping and to keep your knees behind your toe when doing squats.
Good luck!
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