I had originally intended to run Sub 6:00 on a race in December, but I misread the schedule and showed up just after the race ran (instead of an hour before). I decided to go out to a path by the ocean and just time myself, but I was barely able to break 7:00. The conditions weren't great, but that's no excuse for missing by so much and the mile was PAINFUL.
I did however about 2 weeks later run a 5k and ran my first mile in what I guess was about 6:30-6:40 quite comfortably (running with others has always helped me) and ran 22:39 in less than ideal conditions (I had been in vegas for a week and my flight back didn't get in until almost 1am and the race was at 8:30am.
Since then I've been increasing my distances a little for my training runs as I didn't think I had a strong enough base going. I do think that my goal date is no pretty unrealistic because I didn't make as much progress as I was expecting, but I plan on keeping it up and seeing how close I can get with still the eventual goal (hopefully not long after) of breaking 5:00.