@Jonny, I have two boys ages, 5 and 1. I was going to college at night (just graduated college this past may), and work full-time. My wife works full-time as well. I did a 16 week training plan and ran 4 times a week. I ran 3 times during the work week, woke up at 5:00am to get those runs in. For exampe I would run 5 miles monday, wedneday would be a run anywhere between 7-8 miles, and thursday would run 5 miles, then on saturday or sunday would be long run. My first long run started at 13 miles and gradually increased it. I would suggest running two 20 milers in training. No need to run over 20 miles in training, that last 6 miles in a marathon is all mental. Run your long run slow to avoid injury. This is very unconvential, but it works for me.
Cheers!