Every piece of work has two phases. First there’s an uphill phase where you figure out your approach. You have a basic idea about the task, but you haven’t figured out what the solution is going to look like or how to solve all the unknowns.
Eventually you reach a point where there aren’t any more unsolved problems. That’s like standing at the top of the hill. You can see clearly all the way down the other side. Then the downhill phase is just about execution.