Shawn has it right
Shawn has it. You have to treat it as a second job and don't let other people treat your writing time as a resource.
IMHO...
It helps to get some success in early - e.g. a short story in print, or a short novella for a small publisher. That validates your grand project and makes it seem less like you're throwing time away that could be spent with kith and kin.
However, the big thing is to do a very thorough life laundry.
You need to make space for writing without compromising the things that matter. Cut the faffing and the frittering, but not the time spent on your nearest and dearest. :kiss
That can mean dropping hobbies, dependent or non-entertaining friends, not watching TV except for a couple of carefully chosen shows (e.g. I've kept up with Buffy and SG1), not wasting time on the internet, and not getting drawn into activities parallel to writing such as reading groups and fandom etc.
For example, I used to do almost anything to secure company at lunchtime. Now I limit myself to one lunchtime coffee with a mate per week. Other days find me in the local pub, hunched over my laptop. I can manage about 700 words in the time. (It helps me to work from an outline since I can tweak this in odd moments, and think about it without access to a computer.)
It can also mean meeting your general needs in ways that help your writing. I need exercise to fight off the middle aged flab, and to keep my brain going. I also need a social circle of like minded people which exists without too much input from me. So I do western martial arts and battle reenactment. I have mates who talk about swords and armour, am fairly fit, and can write about medieval combat from the inside (I'm off this weekend to spend two days fighting in plate armour). If I wrote military SF, I'd take up paint wars and orienteering instead.
If some of this seems a little selfish, then too bad. When you're old and wrinkly, you won't remember the names of most of your friends, or the TV shows you watched, or the bulletin boards you haunted. But you will be able to point to a creaking shelf of paperback novels with your name on each spine.
You might want to read
www.sff.net/people/jennif...egarb.html
Good luck!