I was intrigued to see this data from my ride today. Average wattage was 208, which is about what I hope to put out in my next IM bike leg. (Adjusted wattage of 230, according to some algorithm that surely accounts for the warm temperature.)
That wattage produced an average speed of 17.6 over a moderately hilly (3250 feet) 50 miles. I anticipate closer to 20.5 mph in an IM race.
There are two obvious differences. First, I was riding a road bike that is set up to be comfortable. (It is comfortable, too. After two weeks of riding the two Cervelos, getting back on the Gunnar felt like settling into a Lazy Boy and grabbing the remote.) Surely there is some aerodynamic penalty. Second, the bike was loaded more or less how it will be in the 600K 10 days from now -- and not much lighter than it will be in the 1200K in July.
My perhaps naive conclusion is that the weight made a real difference in the amount of work required to maintain basic speed, in light of the constant rolling hills. I further conclude that it is easy to understand why holding 18 mph in a randonneuring event is so doggone hard to do.