I actually think this is them running to the pit stop. Why? Thanks to the KTLA preview we know the first teams to arrive to Paraguay did it some time around noon-early afternoon. I remember that day as I had a final. It was really hot. The sun was shining with all its glory and we can see that in the caps of Rachel/Brendon watermelon time.
We then see the two discussing the "Vanessa incident" in the taxi and we can see this is late afternoon. We see Vanessa/Ralph have stayed behind and spent there a lot of time. Look at the shadows in the background in these two pictures:
Then, we see the sun is beginning to set when Rachel does the roadblock. The picture in question with the two of them running and a team chasing them seems to have been taken with less sunlight.
The Plaza de la Democracia roadblock and the Escalinata Antequera pit stop are just 8 blocks apart, so it could easily be a foot race to the mat.
We know from the pit stop picture that some teams checked in at night. Vanessa/Ralph are seen in the picture.
It fits a possible schedule: considering distance and traffic during rush hour, the entire leg could have happened between 4-7 hours, depending on the speed of the teams in the tasks.
Also, some roads in downtown Asuncion are not very wide and have little traffic. That's another reason why I put them somewhere over there. The Metalurgica Punta de Rieles is located in a VERY crowded avenue, but in an area that doesn't have a lot of buildings like the one depicted in the aforementioned picture. Same goes to the university.
But of course, I could be wrong. We'll see it tonight.