“Walk This Way” felt like the best chance Run-D.M.C. had to take a leap, to become rap’s first superstars. And to do that, Profile Records co-owner Steve Plotnicki knew they would have to crack MTV, and not just for an occasional play after midnight. They were going to need to find a slot in the music network’s regular rotation. And that, for a rap song, wasn’t just rare in 1986. It was unprecedented.