I'm reading this thread with interest although I won't be riding the Mille Cymru (too disorganised to plan that far ahead!).
I rode the Maniac Grimpeur a couple of years ago (ride report here:
Maniac Grimpeur, July 2008) & perhaps some of the experience I gained from that may help people prepare for the MC.
I agree with FY that riding hilly 200/300s back to back is really good preparation. It's not just the physical training - it's also the confidence it gives you knowing that you can ride a tough 300, grab a few hours sleep & then tackle another tough ride the following day. A long ride like the MC can seem really daunting unless you break it down into chunks that you know you are capable of doing.
I rode the Maniac in 3 sections: 300 (sleep), 300 (sleep), 400. So I was only riding through the night on the 3rd night. This worked well for me. It looks like the MC can be ridden in a similar fashion.
You certainly don't need to be a fast rider to complete a ride like this successfully (I'm not!). I guess it partly depends on how much sleep you want but I found I managed the Maniac with 3-4 hours a night. Much better, in my opinion, to go at something like this slowly & steadily.
Another piece of advice would be not to plan the ride too tightly. I found it helpful to set a rough timetable for each day on the Maniac, but the one thing you can be certain of on a long ride is that something will go wrong sometime & disrupt your plans. On the Maniac it was breaking my front light on the first day & not being able to get a decent replacement. These things happen - you just have to be a bit flexible. Unless it's really serious it doesn't have to mean the end of your ride.
The really, really important thing is to remember that you're doing this knid of thing because you enjoy it (hopefully). For me (& I suspect many other Audax rider) part of the enjoyment comes from the physical & mental challenge but you shouldn't let the challenge overwhelm the enjoyment. Wales is a wonderful place to be on a bike, the scenery is fantastic, so enjoy it!
I hope some of this is helpful to people.