A study was published, some years ago, that compared the injury rate of runners wearing "high tech" running shoes you find in running stores and those of runners wearing "low tech" running shoes you find in retail stores like Wal-mart. The injury rate was the same. Good luck finding a $100 running shoe at Wal-mart.
Overpronation of the foot is a learned activity. Anything that can be learned can be unlearned. There are also people, the vast majority of runners/walkers in this Runnersworld Nation, who are simply not biomechanically built to run long distances. This includes the vast majority of females (high Q-angle). You have to be fit to run, but you don't need to run to get fit. Basically in order to run long distances without getting hurt, developing shin splints and other ailments, you need to be built a certain way. You need to be pretty biomechanically effecient. If you've gone so far as to compete on a high school team, college team, or continue to run years afterwards then it is safe to assume you are biomechanically effecient enough. I know people who barely creep along at 10 minute pace for 20-30 miles a week who simply cannot go 6 months without something hurting. This is not the fault of their shoes or their training but of their biomechanics.
Our feet are designed to take in information and transfer forces so that one joint or one muscle is not taking in the full force of the impact. The problem with big cushy shoes or stiff shoes is that it interfers with the information sendback and the transfer of forces. By running on a pillow of air or some other soft surface, basically every cushioning "system" out there, you are tricking your brain and the information that is being sent back from your feet. So, the information being sent back is underestimating the impact forces. What happens during the day when you are running along and you run off a steep ledge? You are prepared for it. It is not a jolt. What happens when you do the same running a night and you can not see the ledge? You make a loud and painful thud. By running in shoes with minimal cushioning/support you are strengthening your feet and allowing them to do their job. Your stride begins at your foot. For every action there is an equal and opposite reaction. By allowing your foot to be closer to the ground, by allowing your foot to take on more impact forces you are allowing your foot to send back information and transfer those forces more effectively thereby creating a more powerful stride AND better adaptations to prevent injuries from happening.
If you decide to do all of your running in flats or lightweight trainers it would be wise to do most of your daily activities in such lightweight shoes as well. It does no good to run an hour or two a day in light shoes if you are spending most of the day in big fat shoes.
Alan