A couple of days later, we went to Lowe's and bought an 8-foot length of 2x6" pressure-treated lumber. (The steps are four feet wide and consist of a pair of 2x6s secured to the framework by wood screws.) Lowe's wouldn't cut pressure-treated lumber, but it wasn't too difficult for Lisa to cut the board in half this afternoon. Getting the old wood screws out was fairly difficult, however. First, we had to dig dirt and old paint out of the tops enough to get a screwdriver to fit. Second, eight of the twelve screws were awkwardly placed for using the electric drill. Third, even the four in the center wouldn't come out easily, and Lisa had to use an adapted socket wrench and a pair of Vice-grips to work the recalcitrant wood screws out. I stood by, carried away boards as they were removed, and retrieved the drill bit when it rolled off the step once.
Putting in the new boards, by comparison to the removal of the old ones, was a piece of cake. Interestingly, the unpainted pressure-treated new boards look about the same as the painted (but badly faded) old boards surrounding them. It's a good afternoon's work to get rid of of a step that could have failed under us or at least tripped us up.