If you run regularly it's easy, I started training mid August last year for a two day mountain marathon at the end of oct 2012, within a month I could run 13 miles 2-3 times a week. My best mate who is of average fitness has a half marathon in may, we started running 3 weeks ago, aS I said I would help him train, prior to this he would run only a couple of miles, we've done at least 11 miles twice a week the last two weeks (13 on Monday) so yeah it's fine if you're motivated!
And you will have time, you just choose to spend it doing other things.
Edit: you can take your Christmas hat off now