For those who have osteopenia or who want to prevent bone loss by exercising, is training for and running a marathon a good strategy? Research has shown, surprisingly, that long distance runners tend to have lower bone density than non-runners. It turns out that sprinting shorter distances leads to stronger bones (see article in Runners World)
If you are wanting to make the most of your exercising to build your bones, then training for a marathon is not recommended. In my opinion, shorter, faster runs are more fun, anyway. Sprinting is exhilarating and is actually good for the body. Interval runs keep your heart rate up, providing a good aerobic workout and the pounding that takes place during sprinting stimulates bone growth. So run like the wind for as many years as you can!