Limiting impact on joints from runs

I'm about five weeks out from my first Oly and am trying to limit risk of injury from running.  I've done most of my runs on the asphalt but want to switch over to track or treadmill to lessen impact.  Couple questions:

1. What is generally considered less impact on joints - track (hard rubber at nearby stadium) or treadmill? Those are my most immediately available two alternatives to asphalt.  I assume grass or dirt track would be the best, but not sure I can find long enough stretches of either of those surfaces in my vicinity .

2.  Does it help to change surfaces for different runs (i.e. softest for long run, track for intervals, etc.)?  Does that somehow make your legs less susceptible to injury from repetitiveness on any one surface?

3. I would appreciate any other techniques people use to limit impact during tri training while not losing benefit of run training...thanks