Just remember running is actually a natural movement our body was designed for, and most other sports are not. I think a lot of the injury aspect of running is brought on by the modern lifestyle which is increasingly un-natural, as in sitting in offices, convenience and laziness abound, buying cushioned shoes that also at some point was decided needed to be angled and contoured, etc. It's hard to escape the effects of the modern lifestyle... most of our feet are already permanently disfigured by the time we even play sports in school. But back to the topic, and it has already been said many times, they are different sports. One would not expect to win a bike race never having cycled but a world class runner. Sure general fitness may be increased by any activity, but that doesn't make you great at something else. As primarily a runner I am no where near being really fast cyclist, though I am quite happy with where I am at and that is what matters. I'll never be in the TdF or Olympics so what difference does training hard to be a few mph faster on a bike make in the grand scheme of things? Just enjoy what you can do, a do what you enjoy.