The 12.38 time that the OP got is actually pretty good.
The winner of the first Olympics (in 1896) ran a 12.0, which would be a 12.24 FAT or slower. Silver went to a guy who ran 12.2, or 12.44 FAT. So the OP would have gotten a silver and possibly a gold.
The competitors weren't the best in the world even at that time, but they all were in far better shape than the average person today. Their training wasn't that great, but it was probably better than the OP's.
The OP benefitted from blocks, spikes, and a better running surface, but he, like most newbies, wasn't coached to use them properly. He might even have been better off wearing racing flats or light trainers and not using blocks.