In reply to tingle:
If you run slower then you land more vertically, virtually stop on each stride, and hence more stress on your body.
When you go faster you lean further forward and you simply maintain your forward motion by pushing against the ground.
I always found, when coming back from any injury/layoff, that a shorter run was better, as I could maintain a higher pace. Then once my body was used to it, I would indulge in longer (and hence slower) runs.