It doesn't work like that. Sprinting is completley different to distance running. It's like saying Bolt would be able to swim 400m faster than any of us, or play a better round of golf.
Indeed - Newton has a body type, muscle type and training background which is actually relatively well suited to running marathons (as well as being a track cyclist, where he tended to compete in distance events he also raced in the road quite successfully in races which took longer than a marathon). To be honest 2:59 isn't all that special anyway - I couldn't run that now, but I certainly could have not all that long ago, and even now with a few months training...
Have you seen what sort of times Redgrave and Pinsent run a marathon in?
Bringing Cav and Kittel into the argument is daft. Have you not noticed that part of their job is to cover large distances every day, and to make it over the mountains inside the time limit? Hence they're quite good at that. I wonder how well Sir Chris Hoy would manage on the etape?