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Sorry very poor taste in the title.
How relevant to marathon training is a fartlek session, and if you were going to substitute it what would you add in its place.
Any speedwork can help make you a quicker runner, and adds variety to a weekly schedule. I started using Fartlek for cross country races, but dont see why they cant be done occassionally for long distance runners.
What type of terrain is the marathon on (I'd presume all roads) and are there any undulations / hills? If the answer is yes, then definitely add in a fartlek session, maybe once a fortnight - either increasing speed between lamposts or landmarks (I use trees or bins) or running faster bursts up hills, with a recovery on the flat.
As for alternatives you could try pickups, running timed intervals, such as 5 x 4 mins faster running, with 2-3 mins recovery, or try a tempo run. After a good warm up, run for 20 - 25 minutes at e.g. your 1/2 pace, then have a good warm down jog.
Have a look at some of the suggestions on the top training tab too.
Do you use an mp3?
Have about ten songs that you know well on it. Choose a short circuit that takes you about 4 minutes at your marathon pace, then take exactly the time of each song to complete each circuit. Throw in a short song to really get a sweat on.
The flatness of the course is irrelevant. Fartlek and hill work are not there solely to prepare you for hills, they make you push your body in a different way, and improve a range of things, from lactic threshold to running form.
Doing hills is beneficial whatever the race profile.
I struggle with speedwork outside as can never quite get my distances right. Do you get the same benefit if you do speed work on the treadmill? And what kind of distances are best? As long as 1km fast then slow or something shorter?
First of all, you can train on atreadmill, and it is the only type of training I would consider on a treadmill, as any other run just gets boring. You will not however get the same benefit as from running outdoors, as numerous posts on these forums explain.
As to the type of excercise itself, there are lots of them, I am not going to start long explanations, but direct you to the articles on this site, starting for example with
The key is to vary them.