All my training has been done on roads/sidewalks because I don't have a license and my parents aren't going to wake up at the crack of dawn to drive me to a track or trail. I live in a pretty urban area so we don't really have trails nearby. For the past 4 years, all my running has been on roads and I have never been injured once. I do a pretty decent amount of mileage too - about 50-60 mpw- and I think that the pounding teaches proper form and footstrike. Opinions?