It’s pretty basic.
First of all, I believe that marathon running at these speeds is simply a recreational hobby like any other activity you might take up in your forties. Realistically by today’s standards a three-hour marathoner is basically (other than in our own minds) a jogger. Therefore to keep it exciting and remain motivated what is required is to run lots, have fun, and stay healthy. There is not much more to it than that.
At my peak in results I was running 10 miles one day and 15 the next at about a 8-9 minute mile pace with lots of days off and regular long runs of about 25 miles. My speed work was shorter races.
I submitted a few comments to the Running in the Zone book about ten years ago if you can get a copy of it.
http://www.runninginthezone.ca/blog/category/a-peek-inside-the-covers/