Scared to start Insanity but really really want to!

I had my fourth baby a year ago next week. I completed Insanity in November and moved on to Asylum Vol 1. I'm currently doing a hybrid of the two. I love the workouts and have seen huge improvements! I've lost 15 lbs, almost 6 in off my waist, 5 off the hips and 5 off my chest. It works, but it is hard! Go for it and go at your own pace-- especially since you e had a back injury. You certainly don't need to reinjure yourself. Just commit to it, pace yourself and go for it! If you do it at all, you'll see results!