Variations of this title have often come up before, so I figured I'd do my best to answer the question.
It all started last spring when one of my co-workers wanted to take some time off to go to Rio and watch the Olympics in person. The discussion eventually turned to the 100m and how much an average person who did casual training would lose to the top Olympians.
A consensus couldn't be reached (100m time estimates ranged from mid 11's to low 14's). But I was curious enough to give sprinting a try and see what my potential would be. I was never one of the fastest kids in grade school, nor was I ever built like a sprinter. In high school, I tried out for baseball and basketball but didn't make either team. Cross country was pretty much the only "no cut" sport, so I did that for two seasons before quitting since I was fed up with finishing at the back of the pack.
In the 10 years since I graduated, I ran about five miles a week to stay in shape. I've never had a sprint coach in my life, so Google became my coach. I eventually learned what GPP, SPP, extensive tempo, and all of the other sprint terms were. (Side note - I also stumbled upon on this website).
After doing the conditioning, I hacked together a weekly training schedule. I did three hard days a week - one acceleration day (8x30 or similar), another max velocity day (4x60 or similar), and another speed endurance day (3x150 or similar). The remaining days would be easy days or days off.
A few months of this gave me a surprising amount of speed. By fall, I was getting occasional compliments from random strangers on the track. A few 10-12 year old kids even thought that I was some kind of pro. They would ask questions like "Are you going to the Olympics?" and "Do you think you could beat Usain Bolt?" This was both flattering and a bit embarrassing, to say the least. I went from being a person who couldn't win any footraces to someone who was often the fastest guy on the track.
In December, I got a speed sled to work on my power and acceleration. I also looked into doing deadlifts and squats, but decided that the risk of injury was too high since I would have to learn them from scratch and didn't have anyone to watch my form.
Later, I found that the general public could run unattached in some college meets. For me, this was a chance to experience the college track season I've never had. There were tons of hobby joggers running your local road races, but why were there hardly any hobby sprinters running in these meets? The big meets had strict entry standards, but I was still excited to discover that I could race against guys running for D2 and D3 colleges. They asked for a seed time to enter. I figured that this was probably a best guess time, so I had a friend time my 100. He said I got 11.96, so I entered that time and hoped for the best. I also Googled "block settings" and "how to use blocks". Attempts to actually practice with them mostly proved unsatisfactory.
I showed up to the track an hour before my race and was a bit overwhelmed by the atmosphere and focus of the athletes. The practice area was filled with people doing explosive bounds, strides, and speed drills, many of which I had never even seen before. One pole vaulter's run up (with pole in hand) looked faster than my best all-out sprint.
I was so screwed.
There were 8 people in my heat. Lane 5 had the fastest seed time, so I swore I'd do whatever that guy did. He took a few practice crouch starts, I took a few practice crouch starts. He did a tuck jump before getting into his blocks, I did a tuck jump before getting into my blocks. By the time the gun went off, I wondered whether my warmup and pre-race routine had become my workout, leaving me too tired to run fast in the actual race.
I was "in the zone" for the first few seconds of the race and didn't remember much. About halfway through, I became more aware of my surroundings and wondered what position I was. Miraculously, I wasn't last! I saw a bit of a shadow and heard someone close behind. A few people were not too far ahead of me, which was also a pleasant surprise. I couldn't close the gap but didn't lose any more ground. I finished sixth (of 8) with a time of 12.53 and could have gotten fifth if I ran 0.01s faster. I wondered why I underperformed relative to my seed time before concluding that the difference between me reacting to the gun and my friend reacting to my first movement was more than half a second.
Knowing roughly what to expect (and using a realistic seed time), I was less nervous for my next race a week later. Three people scratched, leaving only me and four other guys in my heat. One guy pulled away comfortably to win in 11.36, but I thought that I might be able to beat the second place guy, who was in the lane next to me and a foot ahead. I ended up finishing third with a 12.40.
For my third race, I looked up info on tapering and did a week-long taper. My workouts felt great, and I felt like an actual track runner for the first time in my life.
The problem was the weather. Race day temps were in the mid 80s, while I was used to training and racing in 50-60 degree weather. Doing the warmup drills and strides was tiring, and it felt like my legs were stuck on the ground and not going anywhere. It got so bad that one of the meet officials asked whether I was OK after I rushed to the shade after a stride and lay down. I even considered scratching from the meet but gave it a shot since I had already driven this far.
I wasn't confident at all, and the fact that someone in my heat got disqualified for a false start didn't help. A second after the gun went off, I was about 2 meters behind the guy on my right, but I closed the gap and passed him by the 30-40 meter mark. I was closing in on the top guy as well, but failed to pass him by the end. I ran a PR of 12.38, and the wind gauge read +0.0. The round number makes me think that this could have been a malfunction, but none of the heats before me or after me had winds greater than 2.0 m/s (my previous two races were both wind-legal).
So that's how my "track season" has turned out. Based on what I did, most people should be able to run the 100 in the low 12's after doing some training. Getting in the mid-high 11's may be possible if they had a coach and really focused on sprinting for a few years. I don't know whether I'll keep doing this, try other events, or quit, but it's certainly been an interesting experience, and I'll keep everyone posted if I run any notable times.
TL; DR: I'm in my late 20s and consider myself to be an average person. A year's worth of decent, but not optimal, sprint training got me to a 12.38 100 (FAT, wind legal).