Every aspect of writing is subject to momentum. Everything. The number of posts you publish, your tendency to have a consistently large (or consistently small) daily output or writing, the number of truly creative ideas you think up, the number of new readers you will attract this year, and so on.
Remember, objects in motion tend to stay in motion. Therefore, when you're "in motion," keep pushing! If you're having a great writing session, keep on writing. Don't cut it short. When the words are flowing, keep them flowing. Stay in motion.
The hard part, always, is getting started. Once you've done that, once you've succeeded in sitting down and actually starting to write, the battle is won.