Have a bike that will manage 100km (which is exactly what it is, although I never intended it that way). It is too cold at night to mess about fixing flats*, gears that won't shift** and loose mudguards, so it would be appreciated if everyone would bring reliable kit, even if that means doing what I did on the Dun Run and riding a frankenfixie with Marathon Plus tyres.
There are some HARD hills that would trouble a TdF peloton, although the majority of the route mileage is actually flat.
Bale-out options involve waiting for the first train of the morning, which is about 0550 from Beaconsfield and probably considerably later from little places like Seer Green or Saunderton. Better options than on the Dun Run, but you might die of hypothermia before the station opens.
* four last time, including a complete tyre disintegration
** OK, that was me once