I wanted to post this question and answer from the beginners forum on Runner's World web site. I think it's an awesome testament to the importance of aerobic base building and thelearnedfoot couldn't have said it better.
"For the last 8 weeks I had followed the 5k program I found on the front page of runnersworld.com and all seemed to be going fine. I haven't really ran to see if my 5k time has dropped, but I'm sure that it has as I've been upping my miles a few at a time week by week and included some speedwork as well.
This is what I have found to be the prevalent line of thought on how to improve on most running sites... simultaneously up your miles a bit at a time, as well as utilizing speedwork once or twice a week.
I was speaking to someone else however who runs a sub 4:20 mile and was told that I should focus almost solely on distance running until I'm up to about 50 miles per week to build a strong base, and then to switch over to speed and strength drills several times a week in order to see my times drop.
I actually really like what he said and felt it made sense. So I'm thinking about switching to easier paced distance runs several times a week to build a strong foundation and skipping the speedwork for now.
Can only speak from my own experience. Last fall, I ran a 5K in 26:20. That was my PR at the time. I was running 35 miles or so per week. Over the late fall/winter, I upped my miles to the point where I was actually averaging 70 and then up to 100 miles per week. This spring, I ran a 5K in 20 minutes and change. I did no speedwork other than an occasional fartlek or a very occasional tempo type run. Nearly all my running was done at an easy, conversational pace.
At first when I switched to the easy paced long distance running, I found myself slower. (Example -- ran a 5K in 27 something in November when I was starting to up the miles) However, it was temporary (obviously).
I am not advocating running as much as I do, but my philosophy on running is runners should try and go as far as they can with the easy base building stuff. How far you go in part depends on how much time you can devote to running and in part how much volume your body can handle (keeping the runs easy is key to upping volume). When you can't go any further and no longer see your times improve with just easy, aerobic base building, then its time to try stuff like speedwork.
(I think the problem a lot of folks have is they don't want to do the easy, base building runs. They would rather skip it because its boring and jump into running fast or long races. Then they burn out or get hurt or don't reach their potential. There's nothing, in my opinion, that can subsitute for having a very strong aerobic base and foundation.)"