In the late 70's and early 80's GM put a clear textured pad rock guard on the fenders of the trucks. That stuff was hard as nails and would work great on the running boards if placed where you step all the time.
I got some peel & stick anti-skid strips for bathtubs at the hardware store. They fit perfect on the top of the running board ridges. I have a bad habit of just jumping in the truck without checking my soles so gravel and dirt is almost always under foot.
