Everyone's different. You definitely couldn't get the PBs Daren's got without the speedwork. If all you do is slow running then you're not going to get any faster but it's getting the right mix of easy and quality to suit the individual. I don't worry about the paces of my long or easy runs now because I know the key to me achieving a faster marathon is the speed work, whereas someone else would have that sorted and need to work more on the HADD type running. Daren, for example if he was marathon training.
We're all just an experiment of 1, as someone said the other day.
I'm heading to the hills now for 7m and to see what conditions are like ahead of a proposed 12m fell race tomorrow.