Yeah, I can easily answer this question. My 5k PR is 14:45 from when I was just running. I am a triathlete and had a long period of just cycling. When I first started running again I was in about 18:00 shape. I highly doubt the TDF guys could run much faster with NO running. However, with very little running, IE 10-20 miles a week along with their cycling, we would see these guys running sub 16 without question.
I can run a low 15 minute 5k off of 20-30 miles a week of running if I am also cycling 6-8 hours of mostly higher intensity and swimming for 3-4 hours. A little running goes a long towards prepping the legs for the musculoskeletal demands of the race.