I used to run a fair amount (~50 miles/week) until a car ran a red light and ran into me. As I was heading for the pavement I remember looking up at the traffic signal and thinking "If I don't survive this, Dan won't know I had the green light and will wonder if this is how I killed myself." At that point I hadn't found a bike that fit me and I got my sweat fix by running. Although I got out of that without any serious injuries, my knees just kept giving out on me every time I got back to 3 miles, and that just wasn't enough for me so I quit. I felt like crying everytime I saw someone running. Ultimately I did go onto daily zoloft, which has been good, but there is one major difference between zoloft and cycling. Zoloft just lets me feel good if I feel good, cycling makes me feel good. For me it takes both.