My training is based on frequent short runs (FSR).
Besides running I have a family and a fulltime job to take care of. Therefore to me it is very important that my training is not too much time consuming.
So as my training has to be quite efficient, I try to cut in to the essential training parameters. I concentrate on the marathon race pace (MP) rather than the distance. I believe intensity is the primary training parameter and I believe that the total volume in MP and faster will increase and have a better response if I split out my training into shorter runs.
Typically I run 7km in the morning with internal app. 3km in MP and 8km in the evening with one or two km in MP. A typical week include 100km over 13 sessions with 30km in MP or faster. Early in my preparations I do some speed in my 5000m race pace. I put in one or two 1km intervals in my regular sessions or I drop in at a Fitness centre and take a specific intensity session on a treadmill. Later in my preparations I concentrate more on my MP training on the middle of the road at 80-90% of my VOmax.
I have more than 20 years of experience with frequent
short runs. When I was a student, I have even tried to
have 3-4 sessions per day, and I realized how powerful it
was to split out my training in more frequent shorter runs.
Since 2002 I have used frequent short runs for marathon
training (Summary: 2001-2005/2006 week 1-15) and I think
a correlation with endurance data of the best Danish
marathon runners show that my Shortcut to Marathon
is an efficient training method.
Postscript June 2011:
From 2001-2009 I did 25 marathons on this kind of training. At that time my training was part of my transportation to and from my place of work. Today I normally run 7-11 km once a day and that is first of all because of my new place of work 40 km further out. Now to get some of the same effect as earlier I often put in some breaks on the way. You may call it FS(FSR)!
FSR - how to get started 
It is the amount of specific training that matters. FSR is an easy way to take up specific training. Specific marathon training is training done at marathon race pace.
"Oh, I want to be that complete
I want to touch the light, the heat
I see in your eyes" - Peter Gabriel
Peder Troldborg
According to Jack Daniels, 2005. Running Formula, 2nd ed.: 39-40, Table 2.2
I score app. 140 intensity points per week.
Take a break!
FSR is just about taking breaks.
It is in the break you recover and where you have the opportunity to improve. So take a break from your training and come back to it later. Just don't let it take too long!