In one sitting (thing)

You sit down. You start something - some task, perhaps a seemingly trivial one but with many intricate complexities, or perhaps one of outright Herculean proportions. You do it. You get up. Then you have done it in one sitting. This has to be an uninterrupted state, mind you - no phone calls, no answering a knock at the door, no pee breaks. In One Sitting is not a successful strategy to winning the 3-day-novel contest. (for winning Everything?)