I'm currently doing 100k training rides on a Mt. Bike and 26x2" nobby tires, and averaging 13.5-13.7 mph
Some of your comments demonstrate that you a have a good understanding of your cycling needs. Since you are a larger rider, selecting a bicycle with a wider gear range and fitting the wheels with at least 28mm tires may be a great hedge against mechanical issues during long cycling events.
Please consider this: selecting a long distance bicycle isn't about how
far you can ride it - its all about how many
hours you can ride it.
For cyclists with less experience, a comfortable handlebar-to-seat-height ratio is important. The "long" road-worthy bicycle-handling supplied by correctly inflated tires with a large enough tread contact patch for stability and comfort is critical. Much less important for the sub 160lb riders......