Fartlek training is good but is best not to do it everyday. I don't believe you mentioned how long you have to train before you run this marathon but the most important thing for you right now is to get a good base. You need to set a few goals for yourself also, such as how fast you plan on running or if you are just doing this to complete it. When training for a marathon you should never do hard sessions on consecutive days. It should be hard, easy, hard, easy, etc... If you are doing 6-7 miles a day right now you should consider doing one long slow run per week of say 10 miles to begin with. Each week increase the distance of that long run until you are up to around 20-21. You should have done at least 2 or 3 of these 20+ mile slow runs by the time the marathon comes around. Also one day a week you should do a workout that will increase your turnover (leg speed). A good example of this would be 12 reps of 800s at your all out 5k pacewith a 400m jog recovery between each. An optimal workout for you would be:
Monday: 6 miles easy
Tuesday: 12 800s on the track or wherever you can
Wednesday:7 miles easy
Thursday: 1 mile warm up 5 miles fartlek 1 mile warm down
Friday: Off
Saturday: 7 miles medium pace
Sunday:10 miles easy
The next week and so on you would want to increase 1/2 a mile to a mile to every workout and increase the pace slightly depending on your conditioning. Try to run 3 or 4 shorter races (5k, 10k, 1/2 marathons) before u actually get to the marathon to get yourself used to the atmosphere and the tension involved in such events.
As far as the supplements go, I would save the ECAs for the Marathon itself. If you use them in training your body will get used to them and they will have little to no effect come race-time. If u wait til about 20 minutes before the marathon to take them you will feel the boost big-time.
Before the long runs (the night before) be sure to take in a high amount of carbs (carboloading).
Unless you are planning on running sub 2:20 I would stay away from EPO, blooding doping, and such. The sides from those are worse than that of Anadrol if not taken properly.
Good luck bro.