I started my journey back to health and fitness with the realization that I didn't really have much of an understanding of food from a nutritional standpoint nor a good working knowledge of how the body uses it. I started with

http://www.amazon.com/Nutrition-Dummies-Carol-Ann-Rinzler/product-reviews/0470932317/ref=sr_1_1_cm_cr_acr_txt?
ie=UTF8&showViewpoints=1

and

http://www.amazon.com/Glycemic-Index...+glycemic+diet

Both are rather good treatments of the subjects that got me started with a basic understanding and helped steer me to other resources and questions. The second title might not seem that helpful at first but once you start to understand how the body processes carbs, it really helps to understand how different foods affect the blood sugar/insulin response relationship. It is a great aid in knowing what carbs to try and avoid or minimize, which work better for recovery, or for in-ride fuel, and more importantly WHY.

A basic understanding of the body's three energy systems is another good essential. You can google some good explanations of this.

There is a good document on training peaks that describes what physiological adaptations can be expected for work performed in what zones.

My suggestions is if you really want to get fit and healthy, start with the nutrition side of the equation. Get plenty of exercise and if that's on the bike do a bit of everything. One or two longer rides a week at a moderate pace mixed in with some shorter more intense efforts on the other days. Make sure you take in some higher GI post workout carbs as close to the completion of exercise as possible(recovery). Get more sleep than usual if at all possible. This is much more important than most people realize.

Good luck. It is possible to make a change. Knowledge is power.