Interesting thread, having just read through most if it.
It is not even vaguely true that there are only 2-3 guys of our age who can run under 17. I live in Montana, not exactly a population center, and can do it, and I can think of others not too much younger than me (I'm 53, and my 5000 PR back in the day was almost identical to yours) who can beat me. So I would not let the skeptics convince you that you cannot run under 17. Most 50 year olds have better things to do with their time and money than go to some age group version of nationals, because we all have people who can kick our asses sideways right here at home.
I would, however, agree with many of the good suggestions that have been made about how to go about racing fast in your 50s. My suggestions--
1. Weight loss and gradual re-entry to training - I took 10 years off of running but gained no weight, which allowed me to bomb back into it but I was 38 at the time. Based on subsequent events, I doubt that would work well after 50, especially with extra weight. I get sore as a freaking dog after running intervals now. (If you want to experience the full splendor (!) of this phenomenon, do some steeple workouts.)
2. Once you're back to looking like a runner, the basic point that everyone has made is very true. Beginning right around 50 or so, it is pretty hard to run your basic "two good workouts and a long run" every week with decent mileage. I am not sure the best reaction to this. Mine is to go ahead a run a tempo, an interval workout and maybe a long run. Based on the ever-increasing ease of beating people my age, I think that the cautious approach that many people advocate will not make you as fast as you could be. Whether you're 20 or 50, you have to go put your nose to the grindstone on Tuesday and Thursday if you want to see what you're actually capable of on Saturday. You're 50, not dead.
3. That said, recovery has to be taken seriously once you're over 50. Easy days easy so hard days can be hard... truer than ever.
4. A few two a days remain your friend, now as before.
5. Someone made a great point about working on power and stride length. We have all seen the old guy out running who's front foot lands overlapping with the one he launched from, and we are slowly becoming him. Form drills and plyos are important to maintain some semblance of knee lift and heel kick. On the other hand, it IS probably possible to speed-walk 17 flat... but people will laugh.
Good luck. I'd not be surprised if you could do it.