I'm not a marathon runner, so my advice may not necessarily help you.
When I last trained for a proper sport, I would do what are called Crossfields. Using an American Football Pitch, I would jog the width of the field. Once I got to the other side, I would sprint back to the original side I was on. I couldn't do many at first, but I got better at it, and it got me into shape pretty quick.
I will say that, when training by running, it's a whole lot easier on your back, knees, ankles, and feet, to train on grass field that's kept up, rather than on cement or tracks. Get in shape on that, and then switch to whatever surface you'll do your marathon on. This way, you spare your body while getting up to par.
Good luck! I don't envy your sport, I hate running and could never do a marathon.