One way to run 20 km is to just run the first 1 km without worrying about the remaining 19. And then repeat that 19 times.
That is how I often run long distances, and I believe, so do a lot of people.
What is true for running, is just as true for any challenging goal in life.
When we worry about everything we have to do, it feels overwhelming. But if we just think about the immediate next task, one task at a time, it becomes doable.
Don’t overthink it – just pick one small thing and do it. Then repeat. Before you realize it, you will be done.