There was reference to this workout on the tv broadcast and I got Salazar to confirm it.
Go to the 2:07 mark. Salazar said 3-4 weeks ago that Rupp ran 20 miles at 4:52 pace and his HR was just 149-50. Healso talks about why Rupp might be better than Mo at the marathon and a few other interesting things.
Here is a 2nd post-race interview with Salazar.