DJC - are you sure you're looking at the right plan? I don't think the 55 miles a week plan goes above 14 for the mid week run, and most of them are around 10 or 12. Sounds more like the 70 mile plan?
That said, it's still a fair distance, but if you plan ahead you can fit them in. Can you include one as a commute to work or back, or combine with another journey? For the hardest week (I did the 12 week plan) which involves 2 x 12 plus 1 x 20 I took a day off work for the second 12, because I just couldn't manage otherwise, but I generally managed to fit the runs in around other stuff.
There's nothing on the website to say it's sold out yet. Oddly, there's no 'enter' button on the front page on the ASO site, you have to to 'running events' and click on the logo from there. I'd sign up soon though, as it does sell out.
Paul, it's a bit of a trek but doable, I've stayed near Parc Montsouris before in the same area. It's not too far from the Expo, and on the day you can take the blue RER then the red to the start - should take about half an hour. There are plenty of hotels a lot closer though, unless you have a particular reason to stay there?