In my search for increasing my marathon speed, I found an article on the Running Times Magazine website that caught my interest! It is titled "Speed Work for Marathoners" by Greg McMillan. The article talks about why a marathoner would include speed work in their training plan, and how to implement it. I have run a couple of those sessions, so far, in my interval sessions. They have been challenging for me, but manageable, as I was able to maintain my target pace. As I'm now in the last 8 weeks of training for NYC, I have incorporated the sample workout into my routine.
Just a word of caution: be sure that you have a good running base, and that you work on endurance over speed first. Also, as Greg states in the article, "These short, fast repeats should not be used, however, for runners who struggle with speed work."