Another vote for Practical Paleo (lots of good information and a large number of really good recipes) - and also for a food diary for a few weeks to get a good idea what you are actually eating. Sometimes we are eating more than we think...there are also so many hidden ingredients in processed foods that it is really hard to know WHAT we are actually eating.

Weight training is important if your doctors allow it. It sounds to me like you will have to tread a fine balance between finding that sweet spot between activity level, fueling properly, and not tweaking your back...