Hi Oldsomeman,
You have some good info in the other posts already: take a look about in the HUBB for more threads about France for instance - do a search for that word for instance.
The thing is that France is a big country by Europes' standards - you could ride further if you were to head for the south of France than you would cover if you did go to Austria.
Don't worry about that tinge of nervousness: it is all part of the trip - much better than going on a package trip with all of the details signed and sealed off by someone else!

When pulling out to ride on the RHS of the road, look for other traffic literally - do whatever the other traffic is doing - mistakes are made when there is no other traffic around and you forget that you are not in the UK (+ when you are tired, as ever). Roundabouts are "interesting" and there are lots of them in France nowadays.
Yep, the whole of France is on vacation in August, so the roads are busy to say the least. I agree with staying off the motorways, but you will then travel less quickly in covering the ground shown on the map i.e. don't over-estimate how far you can get in a set number of days.
Only very rarely have I pre-booked accommodation in France. Even in August you can find somewhere if you look around a bit, don't expect to get into the first place you try and stop to start looking around, say, mid-afternoon. I guess it depends on how fussy you are about what you want to find for accommodation.

That will do for now: have fun on the S/dream!

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Dave