I think it has been a change of mindset. While I may have been too concerned about my times before, I now run based on effort. If I feel like I need to walk during a training run, I'll do it. Paradoxically, I have noticed that I feel less like walking when I am at the end of long runs.
I also get my arms to do more of the work, through creating momentum. The more the arms are moving, the less the legs need to do, so they get tired less easily.
My next marathon is in Valencia in November. I am also doing a half in Nottingham in September.