There was an excellent article last year in "American Track and Field" (I think--it's the free magazine that gets sent to HS coaches every year) that was written by Clyde Hart about training at relatively slow paces. He outlined a bread and butter workout for his 400m types that involves 6-10x200m with about 90 seconds rest. The splits for the 200s were far from impressive for a world-class athlete. However, the repetition with the short rest (for sprinters, that's short rest) results in the last few reps being a race-feel simulation without tearing themselves apart neuromuscularly. Meanwhile, they're building speed endurance as well. For long sprinters, a workout like that is analogous to a tempo run for a distance runner.
Hart freely admitted in the article that he stumbled upon this training by accident rather than design, something about bad weather keeping his team off the outdoor track one year and he had to improvise under less-than-ideal conditions. It was a great article, can't find it online but maybe it's out there somewhere.