I decided to adopt a different training gestalt for my Marathon, this is basically because I thought I wouldn't be able to build up to running for 4 and a half hours in just the 9 weeks that were left when I started it.
What I'm doing now is increasing the amount of time I'm out for each workout (over 3 hours) and adding in a rest period. This gives my body some time to recover before the next 'work phase', and I can stay out longer and get my body acclimatized to being out for the amount of time I will need to.
I am working in blocks of 15 minutes, where 11 minutes is running and 4 is walking. Every week I will increase the workout by 15 minutes and also take 30 seconds off the rest time and add it to the running. By doing this I will (hopefully) at least be able to complete the marathon, even if it involves a bit of walking. Today I did 14 blocks of 15, and did 19.1 miles.
Intervals are also useful for other areas of running training. For instance, if you can already run the distanceyou're aiming for, but you want to improve your time, then you can do intervals of slow and fast rrunning. This will improve your stamina, and help you to run the distance faster. Another useful tip is for when you have to tackle a hilly run, you can do intervals of up and down hills, with some flats mixed in as well.
Viel Spaß homies!