My fastest solo century was an an aluminum-framed KHS CX bike running 32mm file treads. 101.2 miles in 5h16m.
A few years later, I would do 102.0 miles in 5h17m on my Cervelo R3-- which is over five pounds lighter than the KHS, has skinny tires, etc, etc.
Buy whatever you want because you want it. If that serves as motivation, great. I like buying new stuff-- most people do.
But know that the bike is a very small part of the equation. So long as it fits you properly, the amount of speed gained per dollar in buying a fancy CF-everything road bike is so poor it's best not to think about it at all.
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