Admittedly, this is much easier with a solid outline and for an idea/plot/characters that have been living in your head for at least a little while.
I haven't had a chance to do much pure writing lately and did not do it with this challenge either. I am in a period in my freelance career that I have a ton of new cool opportunities that are taking nearly all my time and brainpower. When I do have a little "free" time, I spend that editing my first novel one last time so I can self-publish very, very soon.
However, I 100% believe in adding in this pure writing time, and it is a habit that I did for two months to complete both of my novels. I fully intend to return to this habit when I get my first novel published in the coming weeks to start on book #3.
My only pause is that I like to reach for a word count instead of a number-of-hours-in-a-chair goal. In two hours, I've cranked out between 2,000-3,000 words. Two hours of painfully trying to create perfect sentence after perfect sentence won't get your very far. Churning out a VERY rough draft of 2,000+ words can get you a finished first draft in no time :)