I've done three organized centuries back to back lately, and consequently watched quite a few riders and their bikes. These were non-racing events, which I think are what most of us here participate in, and I did them on eighties steel with friction shifters and 12 gears.
Carbon outnumber all other material at least ten to one. Really fit riders are outnumbered thirty to one. This leads to the already stated conclusion that if you show up in above average shape on a "slower bike", you will easily finish among the faster riders. And probably be more comfortable along the way, too.