There's a discussion on RunningAhead regarding why long slow runs help improve pace. It's definitely worth a read through, but I'll summarize things as I see them.
If you're racing a 5k distance or above, what limits your ability to go "fast" is not your top speed, but the ability to hold close to that speed for the entire race.
You don't develop the capacity to hold your speed by running short distances quickly. You develop aerobic power by learning to run longer. The only way to run longer, consistency, is to slow down!
You can go around your city and find many high school kids who can run 5-minute for a mile. You can have them do all the mile intervals at 5-minutes or faster all you want; but that won't help them run a marathon at 5-minute pace.
This isn't to say that speed isn't necessary, just that speed should be a secondary concern to building your aerobic base. Embrace the power.