I always find it interesting when people say they "hurt" from running too slow. I think the only thing that gets hurt is your ego when on-lookers see you running that slow.
I am still waiting for some science supporting the claim that running too slow "hurts" one's performance.
From my observation, runners that can get away with running 5:45 minute/mile pace on their easy days usually are the low milege types that rely on their above average natural speed.
Yes these runners are successful, much more than I, but it only makes me wonder how much better they could be if they altered their easy runs. For example, instead of a 8-9 mile run at 5:50 pace, why not try a 15-16 mile run at 8 minute pace?
I think the reason why is merely ego. Even if they knew by doing this only a few days a week on easy days that they could improve, many would not be able to overcome the mental stigma of "jogging" at 8 minute pace.
That is what makes the runner competitive I guess, but it is all the more reason to need a good coach to focus that competitive drive.