Endurance racing such as 5k and longer is all about putting in the base and the time. 19.30 is a respectable time, so you've got something good to build on.
So lets break this down. I'm assuming that for your 5k, you're putting in at a minimum 30 miles per week, and have been doing so for some time. Your time is about a 6.17 pace per mile, I'll use that as a reference. You should include two types of workouts, interval and threshold. Intervals are done faster than race pace, with long recovery between. Threshold are done a little slower than race pace, with short recovery. Intervals will increase your top end speed, and threshold workouts will enable you to stay at your race pace longer.
Here is how I would structure a sample week, mid season:
- Monday - Rest day
- Tuesday - 4 miles, steady pace. Aim for around 7, 7:15 per mile pace.
- Wednesday - Interval day: 1-2 mile warmup, 12 x 400, 1 mile warmdown. Your 400's should be all at 1:20 or faster, go every 3 minutes. (That should give you 1:40 to recover between each one). If you find you can't make them, or start to fade the last ones, either increase the time a bit (1:25 or so), or shorten the workout slightly.
- Thursday - 5 miles, steady state
- Friday - 5 miles, steady state, throw in some :30 second pickups (Slowly increase to race pace over 30 seconds) throughout the run. Aim for 5-8 depending on how you feel.
- Saturday - Threshold day: 1-2 mile warmup, 4-6x800 at 3:20 pace, :15-20 seconds recovery. That's a little slower than your race pace. If you can't quite make the last one, increase the recovery a little bit.
- Sunday - 6-10 miles, easy pace, slower than steady state.
The biggest mistake that most uncoached runners make, is that they go way too hard on their steady, easy days and not hard enough on their hard days. And, as noted, that is a mid season type workout. Early season, you should be concentrating on just getting in miles for base, lots of easy pace, steady running. Tapering for races is more art than science, you will have to experiment and know your body to find what kind of taper schedule works for you.
And, each time you run a race, use that time to recalculate everything. Macmillan Running has a nice calculator, and Cool Running has a good pace calculator as well, where you can play with times to see what kind of pacing you would need.