"How to Run a Personal Record: Cover the Ground in Front of You Faster Than Ever Before," by Dave Kuehls, pg. 13-15.
Strength Work exists as a middleman in the training process. It is transitional in nature - getting the runner from Road Work to Track Work - but vital to the overall training plan.
What is it?
Strength Work includes hill climbs and tempo runs.
What does it do for you?
Strength Work gives you strength - in your legs, arms and overall cardiovascular system - strength you will need to "keep the pace" during your race. Strength Work is also a bridge between Road Work and Track Work. By that I mean Strength Work prepares the body (coming from a lot of long, slow runs) for the exertion of track work. Without Strength Work, the abrupt shift from slow to fast could cause an injury that would end the training program.
What workouts does it include?
Several climbs up a gradual hill and tempo runs of 20-40 minutes.
How do I do it?
For hills, start at the bottom of the hill and run up it at a brisk, but controlled pace. For 5Ks and 10Ks, you will do 4-6 repeats. For the half and full marathons, you will do 6-8.
For tempos runs, find a flat stretch of trail of road and run steadily at a pace that 'comfortably close' to PR pace for the distance that you are training for. For 5Ks and 10Ks you will run for 20 minutes. For half and full marathons, you will run for 40 minutes.
What mental approach should I take to Strength Work?
Strength Work is when you really start to focus on effort in your training. You'll need to prepare yourself before each run for the effort and also keep steady during your runs.
What distances for Strength Work should I expect?
Hill Work will amount to a mile or two in total distance; tempo runs of 40 minutes during half and full marathon training can cover four to five miles (6.4 - 8 km).