Dave, how long is the Pau / Tarbes race after the Tours race, and what is the distance from Tours to your loft?
If it was me (and bear in mind the maximum I have raced is 527 miles), I would look at a race of 350 miles maximum, a couple of weeks before the 'BIG' race, then they would be just exercised around the loft upto basketting with possibly a short training spin on the week of basketting.
We have found that to be successful for us upto 527 miles, but whether it would work for bigger distances I am not sure, but if I did race to big distances that is how I would do it.
Hope this helps