In reply to Strachan:
I'd say no one could answer how hard it will be for you as only you will know how you'll cope with the distance. Is there a time limit ? I've ridden quite a few 600km and longer Audaxes, I've always finished them well within the time limit but depending on my fitness levels they can either fell like the hardest thing I've ever done or I can finish them quite comfortably and feel like I could continue riding if I wanted to. The main thing for me on these long rides is to not go into the red at all but just keep it steady and not get tempted to ride with stronger riders especially early on the ride and it may well be that you can get round without maximum fitness by just going slower than you usually would. Being a good cyclist over 150k won't mean you're still strong after 400k.
You don't say what sort of distances you've ridden before but just as important as fitness is comfort on the bike and with your equipment. A saddle that's OK for 200k can become excruciating after 400k, the same for shorts and bike position and things like that you learn as you gradually step up distances, going from 150k to 500k is a big steep learning curve. Apologies if I'm telling you stuff you know and it isn't specific to your ride