Dependng on the size of the main I'd say this migh be a good time to replace it! Have not heard of any records or data being kept on 40yr old CI pipes. DI much the same as CI bt of course much more ductile so the same parameters will not apply.
Another useful point on backfilling any rigid pipe (such as steel) is that the amount of fill is also proportional to the pipe diameter. i.e much more fill can generally be loaded with respect to backfill material on a 200mm dia pipe than a 1.0metre dia pipe without deformation.
