I entered it and did it in 11 hours. I hadn't done any structured training for it although I do have a 28 mile commute each day and do regular 70-80 mile on road every saturday. The bit about mindset is the most important factor I'd say. I knew I was going to finish (without sounding big headed). That helped me during the crap weather and when I was suffering approaching the border crossing. It hurt like hell at that point, but I knew it would end!
It was without a doubt, the best day on a bike I've ever had. I will be signing up as soon as entry opens again. My advice – do the same. Sign up, and just ride plenty. On the day, just get into a rhythm and keep pedalling!! I also rode the last 20 miles with a guy called Roger Stilwell (I think it was Stilwell – if you're on here, give me a shout!!). This helped enormously as well and we had a laugh about how deep the puddles were – anything to pass the time and take your mind off the pedalling!