A few thoughts as a father of 3:
- If your spouse has not bought in to exercise being a high priority in your life, this will be much harder and nearly impossible. Reconsider whether this is the right time for you to run a marathon if this is the case. You don't need to add relationship stress or spousal resentment on top of all of the other stressors in your life.
- If your spouse is supportive, make sure to reciprocate by trying to find 40-60 minutes a day where she has some time to do something she considers a priority. Even a spouse he means to be supportive may end up resentful over time if everything is geared toward your interests during this time. This - my wife deciding that she wanted to start sneaking out for runs to have 30-45 minutes alone just like I did after our 3rd child was born - is how my wife became a runner.
- Be flexible. Some runs and workouts are simply not going to happen. Accept those days and move on to the next one, because your only realistic goal is to limit when that will happen, not eliminate it.
- Be creative. Getting up early to get your runs in is the most obvious choice, but sometimes that may not be an option, so consider things like running home from work, running at lunch time, running late at night after the family is down for the night, running from events, etc. I used to run home from my first kid's soccer games - it would take the family 15 minutes to drive home, so I would be with them for the entire game, my wife would drive them home, and I would show up about 25 minutes after they got home, missing only 25 minutes out of their day, but otherwise being present for the key stuff.
- Don't skimp on sleep (which may mean you will need to be flexible and creative - see above) and be mindful of nutrition. The little things always matter, but they matter even more when you are stretched thin. Illness and injuries can pop up any time, but they are even more likely to pop up during times of high mental and physical stress, and they can derail a training cycle (and can, in the case of illness, impact the rest of your family). Sleep and nutrition are nature's primary preventative medicines.