Well done. Tis a long ways for sure!Got round my first marathon at the weekend, up in Edinburgh. 4hrs 34, woot woot!
As for breathing maybe try a little sprint now and then. That way you will get used to gasping and breathing really hard. When you get back to jogging slowly again you might find you are a bit more relaxed. Be careful though if you haven't done any sprinting for ages you might hurt yourself - just don't go mad.