I did it 4 years ago - it's a great event, lovely roads and a tremendous atmosphere. Make sure you book accomodation for the Sunday night too, so you can drink all day Sunday while you watch the pro race.
Tips - all the various distances share bits of the same routes and there are different coloured signs at junctions. Make sure you follow your sign, not the group you're in or the directions of the marshalls. I was doing the 250km version but ended up finishing after 150km because I'd followed a marshall's directions early on. Did the last 100km loop on my own! The route is mostly easy but there are a few short steep hills. These get very busy with slow cyclists and people walking up the middle. I'd recommend that you have a compact or triple - the lower gearing will help you stay on your bike while you weave around all the walkers and folk on Dutch roadsters and mountain bikes doing 2mph.