I always say, it's very simple, if you want to run this pace in the marathon or the 10k or the 5k, you can't just come from the top down where you just try to do stronger stuff. It's like trying to say, I want to run 26 miles at 5-minute pace, so I'm just going to keep running 30 miles a day at 6-minute pace, then 5:50, then 5:40, 5:30, you know, from the top down. I'm just going to get so strong doing all these pace runs closer and closer to marathon pace, or maybe a little faster than marathon pace or 5k pace. The key is sometimes you've got to attack from the bottom, and do stuff so much frickin' faster that that pace is a jog, and so it costs you less energy. -sportsillustrated.com interview with Alberto Salazar
MY COMMENT: The key of course, how often, how fast, and how long. Just my opinion, but once you've got the endurance, one workout a week should be dedicated to running intervals or tempo faster than your planned race pace. For the marathon, this probably means 800 meter repeats or longer. And for me, the upper limit is 5 miles.