This is only advice on what has worked for me... Have taken my marathon time down from 3'11 to 2'36 over the last two years.
I think of the long run as a session and most weeks have three sessions, Tuesday is VO2Max, Thursday Threshold and Sunday long run.
Regarding the long run, if I am in marathon training, this would quite often include some work at MP, but if not it would always be at a steady effort rather than easy pace. So some example paces for me would be:
Tuesday - trying to hit 5k pace if not faster
Thursday - HMP(ish)
Sunday - 6'50 miles for a steady pace
Everything else is easy pace for me which is whatever I feel is easy on the day, so normally between 7'15 and 8'00 per mile.
I reckon if you focused a bit more on the Sunday run you would see big benefits.