I have congenitally bad knees. Normal human knees have two C-shaped pads of cartilage that gently cradle the rounded bottom of the femur. Each pad is called a meniscus, and the cartilage can hurt a lot if it is damaged. I have discoid menisci, mine are shaped like circles. This means that, instead of creating a nice comfy bowl for the femur to move around in, I have a big flat hunk of cartilage in each knee that gets perpetually ground down by high-impact activities like walking in a bipedal fashion. Surgery did not improve this.
So I have trouble running or doing a lot of physical activities. I also have gastroparesis, a stomach problem that prevents me from eating high fiber foods like fruits, vegetables, and pretty much anything healthy.
Despite my horrible knees and my asthma, I've managed to run and hike several times a week. I stick to unpaved surfaces and wear Vibram Five-Fingers shoes. Recently, I developed exercised-induced acid reflux.
Conclusion: my body wants to be fat.