Makes no sense to get a heavy touring bike designed for carrying heavy loads just so you can ride it unloaded on a supported tour unless you're thinking of getting a heavy loaded touring bike for loaded tours down the line and this is a shakedown ride.
Riding 70+ miles a day doesn't require speed, it only requires sitting on the bike for 6 or so hours. There are a lot of other bikes I'd pick for unloaded riding than a LHT. I wouldn't think of it as more comfortable unless you're seeking a slower handling bike. Can you get 28mm tires on a Roubaix? That'll get you some comfort. Fatter supple tires will be comfy more than a heavy bike with heavy wheels and tires.

If you're concerned about the Roubaix making it just pack it well and if it has racing wheels change them out for training wheels. Is it the logistics of packing and shipping that are a challenge?

If you're greatest concern is protecting the Roubaix for the least amount of money AND getting another bike just for this kind of ride how about this:

http://www.bikesdirect.com/products/...e_xi_steel.htm

replace the 25mm Kendas with 33mm Panaracer Jack Brown tires. It looks like the fork has room, can't tell about the chainstays whether a fatter tire will fit there.

Sounds like you need a beater road bike that can take cushy tires and not a heavy duty touring bike. An old beater road bike made with 531 tubing will still need to be packed carefully so as to not dent the top tube, bend forks or wheels.