I'm GenX and I've never considered the Marlboro Man an icon of my generation. Kurt Cobain is about as close as we'll get on that front.

Westerns were becoming anachronistic by the early 70s, as evidenced by the advent if the anti-western concept (see McCabe and Mrs. Miller) and were being phased out as a prime form of entertainment by the time GenX started coming into its own, so a tired relic of the 50s like the Marlboro Man is hardly representative of a culture that had no real concept of rugged frontier life, such as that evoked by cowboys on horses.