Check out Chi Running. Ultramarathon runner Danny Dryer combined principles from Tai Chi and running technique to form Chi Running - which he's used to run up to 100-mile races. I attended a lecture of his and it really does work - I got faster and didn't get injured as much. Hope it helps... http://www.chirunning.com

Many runneers try to run races with negative splits, meaning the last half of the race is run faster than the first half. To do this, one has to go slower during the first half to conserve energy. Many if not most runners do races with positive splits, meaning they go faster during the first half of the race. The result of a positive split often is that a runner gets quite tired during the last half and ends up walking or jogging some of it.

Here is a link that gives more detail about this.

http://runninginjuryfree.org/2008/09/negative-splits.html

Concerning long distance training runs, run them as LSD (Long Slow Distance). Save your speed work for shorter distances.

http://runninginjuryfree.org/2008/10/long-slow-distance.html