I have a treadmill and had always used it to run, but after having so many injuries it has become hard to run for any distance. Will walking at say 4.3-4.5 miles per hour be just as effective?

Well not as effective from a calorie burning perspective but better for you from a rehab perspective. Running is harsh on the feet and ankles so I prefer speed walking. Try to speed up to 6 mph - but start slow and work your way up to it.
Happy Walking!