A road bike is what you want for TdS, at least if you want to stick with one of the (many) packs and finish at a decent time. I second the recommendation for a reasonably relaxed geometry, smooth riding "century bike". Take a ride on a Roubaix, and you'll see why.

Remember that one of the advantages of drop bars is that you have a variety of hand positions, which helps combat fatigue.

I'd also suggest doing some group rides leading up to TdS, perhaps even one of the "training rides" that are held as the event approaches. Being comfortable in a group will increase your comfort on the ride, allowing you to get the benefit of drafting to speed you along and save energy. "Like" the TdS Facebook page, and you will get periodic updates from the organizer, and notifications of the training rides.

TdS is a well run and challenging ride - have fun!