I think you're way optimistic on how fast "most people" could run.
I'd guess with a year of training, maybe 2 out of 3 twenty-something adults could break 17 seconds. Keep in mind that about a third of the population is obese and couldn't run 20 yards at the start of training and probably an equal amount will get injured in the training process.
Even if the OP was "only" a distance runner in his past athletic experience, and has "only" been running 5 mpw (which says little about being ready to run fast, certainly, but still means you are more active than at least 2/3 of the population), that's going to suggest more potential for running fast than your average Joe who may or may not have gotten off the couch more than 2-3 times a week in the last decade.
I'd assume that the average 100m time for the average adult in the US after a year of pretty decent training would probably be closer to 15 seconds. For only "casual" training (which I would consider to be 2-3 times a week without much structure or plan), probably more like 16-18 seconds, and even that is probably optimistic (such as, either discounting the very obese or those that couldn't train without getting injured within the first couple months and calling it quits).