OK, so stop laughing at the back there.
I'm doing the Dunwich Dynamo in July (120 miles of road from Hackney to Suffolk through the night). And when I recently did the London-Brighton NCN route as a training ride (80 miles all in - longest ride I've done so far), I had only one problem - my arse. Legs fine. Backside so painful I had to ride last few miles standing up.
I used minty arse lard. I had on nice padded shorts. And a BG saddle that I'm very well used to.
On the other side of the coin, it was a very warm day. And I'm a big, hairy bloke.
The bits that hurt were the points where my legs and groin rub together. So I'm thinking hair + sweat + high mileage = ouch.
I'm figuring shaving my arse would cut down on friction. But I'm loathe to do it this close to the ride if it will actually end up hurting more etc.
Anyway, I'm rambling. Bring on the sarcastic comments (and hopefully some actual advice from people who've actually applied a Bic down there before).