While HN is a bit different, I find that a lot of content aggregation sites are big timewasters. (Most notably Reddit.) What does it matter if you have spent 20 minutes composing your life's story (or opinion) when it's going to be downvoted and forgotten in a day or two? That's 20 minutes (plus 3 minutes here and there to check for replies) that you have now wasted. Yes, your opinion matters, yes you are a shining star, a special snowflake... but what matters more, your goal or the satisfaction of showing them that your insignificant opinion matters?
Also, people learn and work best in short intervals. Spend 20 or 40 minutes focussed and then get up and go to the window. Stretch. Don't go to the fridge, you little piggy. Center yourself and still your mind. Flip the album over and go back to work.
Also, setting an unrealistic time goal might just drag you down, as you'll spend more time kicking yourself for having missed it than you will working to achieve it. Set milestones, and they don't have to be perfect. The output doesn't have to be perfect if you can prove to yourself that you've got the socket open and it isn't crashing. Refinement should be it's own milestone, anyway.
You can do it. Don't take my word for it, only your own.