What is the most effective speed training to become a faster long-distance runner?

Hill sprints are a very effective and cheap (meaning low time commitment) method for maintaining a steady neuromuscular connection to your fast-twitch fibers. Find a hill that's very steep, then just sprint, pumping your arms and making sure you're driving your knees up. Each repetition should only last about 10-15 seconds. Five to ten of those per session sprinkled throughout training should keep you sharp.

800 meter track workouts are a great way to do speed work as part of a long distance training program. Both Jeff Galloway and Hal Higdon, two well known running coaches, incorporate these workouts into their training programs. Check out http://halfmarathoners.com for more training resources.

It's going to be a combination of many factors. Obviously you need to be able to run fast but also run fast for a long period of time (judging by your HM and Marathon tags). Below are two parts of the complex equation of running faster. One of the most important factors is increasing your VO2Max. To put it simply, you want to increase the amount of oxygen your heart is able to pump to your muscles by making your heart stronger. This can be done with a variety of repeats in the 400m-1600m zone at a speed faster than your 5k pace. The second, more important for HM and M racing, factor is endurance; how long you can run at a given pace. Assuming you have put in your long runs and have the basic endurance to run a HM/M, the next question is can you run for that distance at a fast pace? This is where the Lactate Threshold run comes in. Take a look here for a introduction: http://runningtimes.com/Article.aspx?ArticleID=21403&PageNum=&CategoryID= . There is a lot of science behind it but, in short, LT runs will be runs in which 3-6 miles are done as close to LT pace as possible. This will adapt your body to deal with lactate accumulation. You will run faster, further.

I don't think there is a training plan as the speed as such is generally not the problem. A fast 5k run wouldn´t be much of a problem but the weak point is the endurance during longer runs. Nutrition is also very important to improve your performance, especially the last two weeks before the competition. You should also do proper  "endurance training" like swimming or other alternative sport that can help you become a faster runner. Especially swimming and cycling are very good to train your muscles.

Generally, long distance runners benefit most from intervals are 3-5 minutes long, with rest intervals that are slightly shorter than that.  The longer the race you run, the more you will benefit from longer intervals, up to the 5 minute limit.  The pace for these intervals should be in the range of your 3K-5K race pace.