Depends on a couple of things as I see it. Some riders probably could (and not necessarily the best).
1 - some people, no matter how "fit" they get will never be great runners because they just don't have the right mechanics to move in the most efficient way. Most runners will know someone who does everything right but is limited in races because they're too big/awkward/inefficient.
2 - what type of rider are you? In the TDF context, you've got a few basic types of rider.
Sprinters e.g. Cavendish who have a specific ability to turn it on for less than 30 seconds when it really matters after a long stage.
GC guys - Froome etc. Strong recovery and very good at riding hard for maybe 40mins at a time when it really matter on the MTFs.
Then a bit of a crossover to some of the classics specialist who can go very fast over a shorter distance - e.g. 10mins or so - someone like Philip Gilbert/Joaquim Rodriguez maybe or even some of the cobbled guys like Van Avermaet or Sagan who can put in huge bursts over shorter distances.
Domestiques - your ultra guys - lower intensity sustained work over many hours.
I think you're probably looking at someone in the GC/hilly classics guys - those who can go ballistic for 15mins at the end of a stage, particularly the lighter, smaller ones.
But better still - cyclo cross guys - used to high intensity 45-60min races with some of it on foot.
Not sure what the potential time might be but I've seen a couple of completely untrained runners get down to 15mins 5k within a few months - real talented guys. I'm sure that there'll be trained cyclists with the right attributes who could manage that but probably not all that many. The running efficiency/style challenge is significant.