Why am I overweight?

In the past few years, I have become increasingly overweight. This excess weight has caused me to develop several physical problems that have made life uncomfortable for me. My knees hurt most of the time. I know it's from carrying around all this extra weight. One knee hurts when walking, when standing, and after sitting for a long time. In other words, it always hurts. Stiffness every morning gets worse all the time. 

I don't know when it was that I noticed I was short of breath most of the time. Just walking to the mailbox felt like a jog to me. I knew this was bad, so I started walking every day. I figured surely I'd lose weight by walking. It was such a chore to me, but I made myself walk for an hour every day.  After my walk, I was so exhausted that I had to rest for the remainder of the day. The knee pain was unbearable. I took Alleve twice a day and still needed to apply a pain relieving cream to the back of my knees.

I was surprised to find that I did not lose any weight walking for a whole hour every day. "What am I doing wrong?"  I wondered. I promised myself I would get to the bottom of this.