Just to add a note of caution to comments like "If you can do a UK 600..."
I wasn't nearly prepared enough for PBP2007. I'd been riding audaxes for a couple of years and I got myself round the BCM600 as my qualifier with four or so hours in hand and felt pretty confident at the time. I was riding every weekend, either on audaxes or with my club. I was strong and I thought I was fit.
PBP came as a huge shock to my system. I was too heavy, not fit enough and I was probably carrying too much kit. The weather (as I'm sure you know) was pretty bad, but it wasn't this that was the problem for me. I found the sheer lack of sleep extraordinarily hard.
There's a hell of a lot more queuing to be done at the controls and perhaps I've had been faster I'd not have had this as bad, but as it was, I had difficulty in building up much of a time buffer. This meant that I was getting by on a couple of hours sleep a night and after the first half of the ride, this started to really make life tough.
You can pretty much manage a 600 with just a brief nap or two, but ninety hours is too long to go without a proper REM sleep. In order to get this, you need to be fast enough to both build up a time buffer and beat the queues to the sleeping facilities. Perhaps if it is beautiful summer weather, you can avoid this by sleeping in hedges. In '07, this wasn't happening.
So my main advice is, "get faster". If at all possible, get lighter and get fitter, too, but unless you're Hummers, these are necessary steps in order to speed up. I'm definitely not ruling out the possibility that I'll be back for another helping in 2011 (despite what I said 89h into PBP2007). If I do; I'm taking my training a lot more seriously because if I ever do The French Ride again, I want to enjoy it.
I will also invest in a bike fit session because what didn't show up on a 600 did on a 1200...