re: 10-week 5k training plan (under 20min)

If you live near a track I suggest getting out there and working on some speed up tempo stuff 2-3 times a week. Long runs and running the race course are only going to do you so good. My tip would be something like this:
Monday: long run - something above 5K. I'd try and at least hit 4-5 miles or more if you can.
Tuesday: speed - 1 mile warm up then hit a track and do 4x800M at a pace much faster than your target. Also add in some weight training. 1 mile recovery run after.
Wednesday: recovery - 3 miles at a good uptempo pace
Thursday: more speed - try doing mile repeats at your race pace or a little faster with light rest between each mile. Lift weights if you can.
Friday: another recovery run day. 3-4 miles up tempo.
Sat/Sun: try and do something active but doesn't have to be a run. (Weights, swim, etc)

Not saying this will get you to your goal but you need to mix it up. Start working at tempos below your goal pace. Don't just run the course everyday or you'll get burned out. Mix up the speed workouts incorporating x400m, x800m, x1200m, x1600m, etc. you'll see better results.

ETA:
5k PR: 17:25
Mile PR: 4:44