Shin splints can definitely lead to fractures. There are a variety of causes of shin splints: not warming up properly, doing too much too soon, your stride being too long, not cooling down, not stretching, weak muscles. There are a couple of good sites that have information: coolrunning, exrx.net, thewalkingsite, etc.
I agree that walking is an excellent exercise and certainly doesn't impact your feet and legs the way running does. I also like to use dvds a few times a week, too.
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