Well Von Braun said that towards the end of the Apollo program. I wonder what ideas he had to get there and if they were more sophisticated than are current plans.
He was a truly brilliant man so might not have written anything down.
He had just gone from suborbital rocketry to the moon in ten years, so I suspect he did not have a specific plan for Mars, but expecting the extraordinary funding to continue he and his team could get it done in ten years. Of course the funding came to a crashing halt.
He had some plans - as highlighted by his 1950's Disney presentations - but I suspect these were a little impractical - especially the time it would have taken to get there.
As with Apollo his creative genius might have come up with something.
My 1970 space annual says will be there in 1985...oh wait a minute.