Well, as I've written elsewhere, I've done it. I know ZZ to be a hardy soul, and will read his accounts of his bicycling exploits with relish, but, to be honest, if I go again I may well leave the bike at home. Bloody terrifying. And this from someone who treats the A3 with disdain.
If you stick to the back roads you'll get very very fit very quickly. The hills are steep. I'd taken the 12/21 off the back and put on a 12/27 - and needed every cog.