I think that as he should have a good level of base fitness, he could do it. But I do think he's seriously underestimating how hard it will be.
I mean, I can do an imperial century in a day (at my leisurely pace) and it's not my 4r$3 that's sore, as my saddle fits, is set at the right height & my cycle shorts are comfy, but my legs do ache... And considering that they are cycling the direction of what is into the prevailing headwind direction...
Adrian Chiles should do it as he's a regular cyclist, but if what Shearer is saying is true, he's understimating. Especially if the weather is less than perfect.