This was his waiting area for the start of the race. (He is about six people to the left from the blue & white "asics" banner. He is actually behind the yellow & green sign held by another runner, so we can't see him.)
We followed his course and cheered him on near the Temple of Hercules, the Ara Pacis (housed in a building designed by Richard Meier), and on the Via del Corso near the Spanish Steps.
Here he is, in the red shirt, just after passing the Ara Pacis.