No excuses. Start writing. Make time to write. That could be early morning before you go to work or get the children up, lunchtimes, evenings or just a couple of hours at the weekends, but you must write. Rather obvious, I know. When I was working full time and running my marketing and training business I could only write on Saturday and Sunday afternoons. But I was fortunate to be doing that and religiously I would write, because I loved doing so. I didn't have a word count. I just did the best I could for that day.
Write the first draft quickly
The first creative draft is often the most enjoyable and the most painful because you have so much in your head that you want to get down on to paper or computer screen and yet you might have gaps in the plot and in character development. I also like to research my marine mystery crime novels as I write so that will hold things up a bit. However, the key is to get an outline down as quickly as possible probably within four months. This will be a very rough draft with incorrect spellings, missing words and the character motivations and descriptions not fully formed. The key is in letting the creative juices flow, just brain dumping them on to the page.
Don't give up on it
You'll have good days and bad days but don't give up. Set yourself a goal to finish that first draft no matter what. Also give yourself a time frame: four months, a year, two, whatever suits you. If you get stuck either write through it, go for a walk, do some gardening or cleaning, knitting, sewing or anything you enjoy but which also lets your mind continue working in the background. Then return to your writing. And keep writing.